1
LTC1626
Low Voltage, High Efficiency
Step-Down DC/DC Converter
FEATURES
DESCRIPTIO
N
U
The LTC
1626 is a monolithic, low voltage, step-down
current mode DC/DC converter featuring Burst Mode
TM
operation at low output current.
The input supply voltage range of 2.5V to 6V makes the
LTC1626 ideal for single cell Li-Ion and 3- or 4-cell NiCd/
NiMH applications. A built-in 0.32
switch (V
IN
= 4.5V)
allows up to 0.6A of output current.
The LTC1626 incorporates automatic power saving Burst
Mode operation to reduce gate charge losses when the
load current drops below the level required for continuous
operation. With no load, the converter draws only 165
A.
In shutdown, it draws a mere 0.5
A--making it ideal for
current sensitive applications.
The inductor current is user-programmable via an external
current sense resistor. In dropout, the internal P-channel
MOSFET switch is turned on continuously, maximizing
battery life.
TYPICAL APPLICATIO
N
U
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
Figure 1. High Efficiency 2.5V Step-Down Converter
OUTPUT CURRENT (A)
0.01
EFFICIENCY (%)
100
95
90
85
80
75
70
0.1
1
1626 F01a
V
IN
= 3.5V
L1 = 33
H
V
OUT
= 2.5V
R
SENSE
= 0.1
C
T
= 270pF
Efficiency
+
PWR V
IN
V
IN
SGND
D1
MBRS130LT
0.1
F
* COILTRONICS CTX33-4
** IRC 1206-R100F
AVX TPSD476KO16
AVX TPSC107M006R0150
C
T
270pF
470
1000pF
3900pF
10k
1626 F01
10k
100pF
L*
33
H
R
SENSE
**
0.1
V
OUT
2.5V
0.25A
C
OUT
100
F
6.3V
V
IN
2.7V TO 6V
C
IN
47
F
16V
SW
LTC1626
SHDN
PGND
SENSE
+
SENSE
I
TH
C
T
V
FB
+
s
Single Cell Li-Ion Step-Down Converters
s
3- or 4-Cell NiMH Step-Down Converters
s
Cellular Telephones
s
5V to 3.3V Conversion
s
3.3V to 2.5V Conversion
s
Inverting Converters
s
Portable Instruments
APPLICATIO
N
S
U
s
Wide Input Supply Voltage Range: 2.5V to 6V
s
High Efficiency: Up to 95%
s
Low R
DS(ON)
Internal Switch: 0.32
(V
IN
= 4.5V)
s
Current Mode Operation for Excellent Line and Load
Transient Response
s
Short-Circuit Protected
s
Low Dropout Operation: 100% Duty Cycle
s
Built-In Low-Battery Detector
s
Low Quiescent Current at Light Loads: I
Q
= 165
A
s
Ultralow Shutdown Current: I
Q
= 0.5
A
s
Peak Inductor Current Independent of Inductor Value
s
Available in 14-Pin SO Package
2
LTC1626
ABSOLUTE
M
AXI
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M
RATINGS
W
W
W
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(Voltages Referred to GND Pin)
Input Supply Voltage (Pins 1, 2, 13) ............ 0.3V to 7V
Shutdown Input Voltage (Pin 10) ................ 0.3V to 7V
Sense
, Sense
+
(Pins 7, 8)........... 0.3V to (V
IN
+ 0.3V)
LBO, LBI (Pins 3, 4) .................................... 0.3V to 7V
C
T
, I
TH
, V
FB
(Pins 5, 6, 9) ............. 0.3V to (V
IN
+ 0.3V)
DC Switch Current (Pin 14) .................................... 1.2A
Peak Switch Current (Pin 14) ................................. 1.6A
Switch Voltage (Pin 14) .......(V
IN
7.5V) to (V
IN
+ 0.3V)
Operating Temperature Range ..................... 0
C to 70
C
Extended Commercial Operating
Temperature Range (Note 4) ............. 40
C to 85
C
Junction Temperature (Note 1) ............................. 125
C
Storage Temperature Range ................. 65
C to 150
C
Lead Temperature (Soldering, 10 sec) .................. 300
C
PACKAGE/ORDER I
N
FOR
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ATIO
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Consult factory for Industrial and Military grade parts.
ORDER PART
NUMBER
TOP VIEW
S PACKAGE
14-LEAD PLASTIC SO
1
2
3
4
5
6
7
14
13
12
11
10
9
8
PWR V
IN
V
IN
LBO
LBI
C
T
I
TH
SENSE
SW
PWR V
IN
PGND
SGND
SHDN
V
FB
SENSE
+
LTC1626CS
T
JMAX
= 125
C,
JA
= 110
C/ W
ELECTRICAL CHARACTERISTICS
T
A
= 25
C, V
IN
= 4.5V, V
OUT
= 2.5V, V
SHDN
= 0V, unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
FB
Feedback Pin Current
0.1
1
A
V
FB
Feedback Voltage
0
C to 70
C
q
1.22
1.25
1.28
V
40
C to 85
C
q
1.2
1.3
V
V
OUT
Output Voltage Line Regulation
V
IN
= 3.5V to 5.5V, I
LOAD
= 250mA
40
0
40
mV
Output Voltage Load Regulation
10mA
I
LOAD
250mA
25
50
mV
Burst Mode Output Ripple
I
LOAD
= 0
50
mV
P-P
I
Q
Input DC Supply Current (Note 2)
Active Mode
q
1.9
3.0
mA
Sleep Mode
q
165
300
A
Shutdown
V
SHDN
= V
IN
0.5
5
A
V
LBTRIP
Low-Battery Trip Point
1.15
1.25
1.35
V
I
LBI
Low-Battery Input Bias Current
0.5
A
I
LBO
Low-Battery Output Sink Current
V
LBO
= 0.4V
0.4
1.4
mA
V
SENSE
Current Sense Threshold Voltage
V
SENSE
= 2.5V, V
FB
= V
OUT
/2 + 25mV (Forced)
25
mV
V
SENSE
+
V
SENSE
V
SENSE
= 2.5V, V
FB
= V
OUT
/2 25mV (Forced)
130
155
180
mV
R
ON
ON Resistance of Switch
0.32
0.45
t
OFF
Switch Off-Time (Note 3)
C
T
= 390pF, I
LOAD
= 400mA
4
5
6
s
V
IHSD
SHDN Pin High
Minimum Voltage for Device to Be Shut Down
V
IN
0.4
V
V
ILSD
SHDN Pin Low
Maximum Voltage for Device to Be Active
0.4
V
I
INSD
SHDN Pin Input Current
0V
V
SHDN
7V
q
1
A
The
q
denotes specifications that apply over the specified operating
temperature range.
Note 1: T
J
is calculated from the ambient temperature T
A
and power
dissipation according to the following formula:
T
J
= T
A
+ (P
D
110
C/W)
Note 2: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 3: In applications where R
SENSE
is placed at ground potential, the
off-time increases by approximately 40%.
Note 4: C grade device specifications are guaranteed over the 0
C to 70
C
temperature range. In addition, C grade device specifications are assured
over the 40
C to 85
C temperature range by design or correlation, but
are not production tested.
3
LTC1626
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
INPUT VOLTAGE (V)
100
98
96
94
92
90
88
86
84
82
80
EFFICIENCY (%)
1626 G01
5.0 5.5 6.0 6.5 7.0 7.5
4.5
2.5
3.5 4.0
3.0
I
OUT
= 100mA
L1 = 33
H
R
SENSE
= 0.1
C
T
= 270pF
I
OUT
= 250mA
INPUT VOLTAGE (V)
100
98
96
94
92
90
88
86
84
82
80
EFFICIENCY (%)
1626 G03
5.0 5.5 6.0 6.5 7.0 7.5
4.5
2.5
3.5 4.0
3.0
I
OUT
= 100mA
L1 = 33
H
R
SENSE
= 0.1
C
T
= 270pF
I
OUT
= 250mA
OUTPUT CURRENT (A)
0.01
EFFICIENCY (%)
100
95
90
85
80
75
70
0.1
1
1626 G02
L1 = 33
H
V
IN
= 5V
V
OUT
= 3.3V
R
SENSE
= 0.1
C
T
= 270pF
Efficiency vs Input Voltage
(V
OUT
= 2.5V)
Efficiency vs Input Voltage
(V
OUT
= 3.3V)
INPUT VOLTAGE (V)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
NORMALIZED FREQUENCY
1626 G04
5.0 5.5 6.0 6.5 7.0 7.5
4.5
2.5
3.5 4.0
3.0
FIGURE 1 CIRCUIT
Switch Resistance
INPUT VOLTAGE (V)
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
R
DS(ON)
(
)
1626 G05
5.0 5.5 6.0 6.5 7.0
2.0
4.5
2.5
3.5 4.0
3.0
T
J
= 70
C
T
J
= 0
C
T
J
= 25
C
JUNCTION TEMPERATURE (
C)
100
90
80
70
60
50
40
30
20
10
0
LEAKAGE CURRENT (
A)
1626 G06
50 60 70 80 90 100
40
0
20 30
10
V
IN
= 4.5V
Switch Leakage Current
Operating Frequency
INPUT VOLTAGE (V)
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
SUPPLY CURRENT (mA)
1626 G07
5.0 5.5 6.0 6.5 7.0 7.5
4.5
2.5
3.5 4.0
3.0
T
J
= 25
C
* DOES NOT INCLUDE
GATE CHARGE CURRENT
ACTIVE MODE
SLEEP MODE
INPUT VOLTAGE (V)
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
SUPPLY CURRENT (
A)
1626 G08
5.0 5.5 6.0 6.5 7.0 7.5
4.5
2.5
3.5 4.0
3.0
T
J
= 25
C
SHUTDOWN = V
IN
INPUT VOLTAGE (V)
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
OUTPUT VOLTAGE (V)
1626 G09
2.5 3.0 3.5 4.0 4.5 5.0
2.0
0
1.0 1.5
0.5
L1 = 33
H
R
SENSE
= 0.1
C
T
= 270pF
T
J
= 25
C
I
LOAD
= 250mA
V
OUT
= 3.3V
V
OUT
= 2.5V
Supply Current in Shutdown
Low Voltage Behavior
DC Supply Current*
Efficiency vs Output Current
(V
OUT
= 3.3V)
4
LTC1626
PWR V
IN
(Pins 1, 13): Supply for the Power MOSFET and
Its Driver. Decouple this pin properly to ground.
V
IN
(Pin 2): Main Supply for All the Control Circuitry in
the LTC1626.
LBO
(Pin 3): Open-Drain Output of the Low-Battery Com-
parator. This pin will sink current when Pin 4 (LBI) goes
below 1.25V. During shutdown, this pin is high imped-
ance.
LBI
(Pin 4): The () Input of the Low-Battery Comparator.
The (+) input is connected to a reference voltage of 1.25V.
If not used, connect to V
IN
.
C
T
(Pin 5): External capacitor C
T
from Pin 5 to ground sets
the switch off-time. The operating frequency is dependent
on the input voltage and C
T
.
I
TH
(Pin 6): Feedback Amplifier Decoupling Point. The
current comparator threshold is proportional to Pin 6
voltage.
PI
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CTIO
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SENSE
(Pin 7): Connects to the () Input of the Current
Comparator.
SENSE
+
(Pin 8): The (+) Input to the Current Comparator.
A built-in offset between Pins 7 and 8 in conjunction with
R
SENSE
sets the current trip threshold.
V
FB
(Pin 9): This pin serves as the feedback pin from an
external resistive divider used to set the output voltage.
SHDN (Pin 10): Shutdown Pin. Pulling this pin to V
IN
keeps the internal switch off and puts the LTC1626 in
micropower shutdown. If not used, connect to SGND.
SGND (Pin 11): Small-Signal Ground. Must be routed
separately from other grounds to the () terminal of C
OUT
.
PWR GND (Pin 12): Switch Driver Ground. Connects to
the () terminal of C
IN
.
SW (Pin 14): Drain of the P-Channel MOSFET Switch.
Cathode of the Schottky diode must be connected closely
to this pin.
BLOCK DIAGRA
M
W
+
+
12
14
1
13
SW
PWR GND
C
+
G
S
+
T
+
+
A3
V
Q
SLEEP
V
TH2
V
FB
V
FB
V
TH1
V
IN
SENSE
SENSE
SENSE
+
25mV TO 150mV
OFF-TIME
CONTROL
C
T
5
I
TH
LBO
LBI
1626 BD
V
OS
SHDN
SGND
13k
+
6
3
10
9
7
8
4
11
2
R
S
PWR V
IN
REFERENCE
5
LTC1626
OPERATIO
U
The nominal off-time of the LTC1626 is set by an external
timing capacitor connected between the C
T
pin and ground.
The operating frequency is then determined by the off-
time and the difference between V
IN
and V
OUT
.
The output voltage is set by an external divider returned to
the V
FB
pin. A voltage comparator V and a gain block G
compare the divided output voltage with a reference
voltage of 1.25V.
To optimize efficiency, the LTC1626 automatically switches
between continuous and Burst Mode operation. The volt-
age comparator is the primary control element when the
device is in Burst Mode operation, while the gain block
controls the output voltage in continuous mode.
When the load is heavy, the LTC1626 is in continuous
operation. During the switch "ON" time, current compara-
tor C monitors the voltage between the SENSE
+
and
SENSE
pins connected across an external shunt in series
with the inductor. When the voltage across the shunt
reaches the comparator's threshold value, its output sig-
nal changes state, resetting the flip-flop and turning the
internal P-channel MOSFET off. The timing capacitor
connected to the C
T
pin is now allowed to discharge at a
rate determined by the off-time controller.
When the voltage on the timing capacitor has discharged
past V
TH1
, comparator T trips, sets the flip-flop and causes
the switch to turn on. Also, the timing capacitor is
recharged. The inductor current will again ramp up until
the current comparator C trips. The cycle then repeats.
When the load current increases, the output voltage
decreases slightly. This causes the output of the gain stage
(Pin 6) to increase the current comparator threshold, thus
tracking the load current.
When the load is relatively light, the LTC1626 automati-
cally switches to Burst Mode operation. The current loop
is interrupted when the output voltage reaches the desired
regulated value. The hysteretic voltage comparator V trips
when V
OUT
is above the desired output voltage, turning off
the switch and causing the timing capacitor to discharge.
This capacitor discharges past V
TH1
until its voltage drops
below V
TH2
. Comparator S then trips and a sleep signal is
generated. The circuit now enters into sleep mode with the
power MOSFET turned off. In sleep mode, the LTC1626 is
in standby and the load current is supplied by the output
capacitor. All unused circuitry is shut off, reducing quies-
cent current from 1.9mA to 165
A. When the output
capacitor discharges by the amount of the hysteresis of
the comparator V, the P-channel switch turns on again and
the process repeats itself. During Burst Mode operation,
the peak inductor's current is set at 25mV/R
SENSE
.
To avoid the operation of the current loop interfering with
Burst Mode operation, a built-in offset V
OS
is incorporated
in the gain stage. This prevents the current from increas-
ing until the output voltage has dropped below a minimum
threshold.
In dropout, the P-channel MOSFET is turned on continu-
ously (100% duty cycle) providing low dropout operation
with V
OUT
V
IN
.
APPLICATIO
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The basic LTC1626 application circuit is shown in Figure
1. External component selection is driven by the load
requirement and begins with the selection of R
SENSE
. Once
R
SENSE
is known, C
T
and L can be chosen. Next, the
Schottky diode D1 is selected followed by C
IN
and C
OUT
.
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
With the current comparator monitoring the voltage devel-
oped across R
SENSE
, the threshold of the comparator
determines the peak inductor current. Depending upon
the load current condition, the threshold of the compara-
tor lies between 25mV/R
SENSE
and 150mV/R
SENSE
. The
maximum output current of the LTC1626 is:
I
OUT(MAX)
= 150mV/R
SENSE
I
RIPPLE
/2 (A)
Where I
RIPPLE
is the peak-to-peak inductor ripple current.
At a relatively light load, the LTC1626 is in Burst Mode
operation. In this mode, the peak current is set at 25mV/
R
SENSE
. To fully benefit from Burst Mode operation, the
6
LTC1626
APPLICATIO
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inductor current should be continuous during burst peri-
ods. Hence, the peak-to-peak inductor ripple current must
not exceed 25mV/R
SENSE
.
To account for light load conditions, the I
OUT(MAX)
is then
given by:
I
OUT(MAX)
= 150mV/R
SENSE
25mV/2R
SENSE
(A)
= 137.5mV/R
SENSE
(A)
Solving for R
SENSE
and allowing a margin of variations in
the LTC1626 and external component values yields:
R
SENSE
= 100mV/I
OUT(MAX)
(
)
The LTC1626 switch is capable of supplying a maximum
of 1.2A of output current.
Therefore, the minimum value
of R
SENSE
that can be used is 0.083
. A graph for
selecting R
SENSE
versus maximum output current is given
in Figure 2.
Operating Frequency Considerations
For most applications, the LTC1626 should be operated in
the 100kHz to 300kHz range. This range can be extended,
however, up to 600kHz, to accommodate smaller size/
valued inductors, such as low profile types, with a slight
decrease in efficiency due to gate charge losses. Some
experimentation may be required to determine the opti-
mum operating frequency for a particular set of external
components and operating conditions.
C
T
and L Selection
The value of C
T
is calculated from the desired continuous
mode operating frequency:
C
V
V
V
V
V
V
f
F
T
IN
OUT
IN
D
IN
BE
O
=
-
(
)
+
(
)( )
-
(
)( ) ( )
3300
where V
D
is the drop across the Schottky diode D1 and V
BE
is a base-emitter voltage drop (0.6V).
The complete expression for operating frequency is given
by:
f
t
V
V
V
V
Hz
O
OFF
IN
OUT
IN
D
-
+
( )
1
where:
t
C
V
V
OFF
T
IN
BE
=
( )( )
-
(
) ( )
3300
sec
Figure 3 is a graph of operating frequency versus power
supply voltage for the 2.5V regulator circuit shown in
Figure 1 (C
T
= 270pF). Note that the frequency is relatively
constant with supply voltage but drops as the supply
voltage approaches the regulated output voltage.
To maintain continuous inductor current at light load, the
inductor must be chosen to provide no more than 25mV/
R
SENSE
of peak-to-peak ripple current. This results in the
following expression for L:
L
R
SENSE
( )
(
)( )( ) ( )
5 2 10
5
.
C
V
H
T
REG
Using an inductance smaller than the above value will
result in inductor current being discontinuous. As a con-
MAXIMUM OUTPUT CURRENT (A)
0.5
0.4
0.3
0.2
0.1
0
R
SENSE
(
)
1626 F02
0.6
0.8
1.0
0.4
0
0.2
Figure 2. Selecting R
SENSE
During a short circuit of the regulator output to ground, the
peak current is determined by:
I
SC
= 150mV/R
SENSE
(A)
In this condition, the LTC1626 automatically extends the
off-time period of the P-channel MOSFET switch to allow
the inductor current to decay far enough to prevent any
current buildup. The resulting ripple current causes the
average current to be approximately I
OUT(MAX)
.
7
LTC1626
sequence, the LTC1626 will delay entering Burst Mode
operation and efficiency will be degraded at low currents.
APPLICATIO
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the P-channel switch duty cycle. At high input voltages,
the diode conducts most of the time. As V
IN
approaches
V
OUT
, the diode conducts only a small fraction of the time.
The most stressful condition for the diode is when the
regulator output is shorted to ground.
Under short-circuit conditions, the diode must safely
handle I
SC(PK)
at close to 100% duty cycle. Most LTC1626
circuits will be well served by either an MBRM5819 or an
MBRS130LT3. An MBR0520LT1 is a good choice for
I
OUT(MAX)
500mA.
Input Capacitor (C
IN
) Selection
In continuous mode, the input current of the converter is
a square wave of duty cycle V
OUT
/V
IN
. To prevent large
voltage transients, a low effective series resistance (ESR)
input capacitor must be used. In addition, the capacitor
must handle a high RMS current. The C
IN
RMS current is
given by:
I
I
V
V
V
V
A
RMS
OUT
OUT
IN
OUT
IN
-
(
)
[
]
( )
1 2
/
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst case is commonly used
to design because even significant deviations do not offer
much relief. Note that capacitor manufacturer's ripple
current ratings are often based on only 2000 hours life-
time. This make it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required.
Do not underspecify this component. An addi-
tional 0.1
F ceramic capacitor is also required on PWR V
IN
for high frequency decoupling.
Output Capacitor (C
OUT
) Selection
The selection of C
OUT
is driven by the ESR for proper
operation of the LTC1626. The required ESR of C
OUT
is:
ESR
COUT
< 50mV/I
RIPPLE
where I
RIPPLE
is the ripple current of the inductor. For the
case where the I
RIPPLE
is 25mV/R
SENSE
, the required ESR
of C
OUT
is:
Kool M
is a registered trademark of Magnetics, Inc.
INPUT VOLTAGE (V)
200
180
160
140
120
100
80
60
40
20
0
FREQUENCY (kHz)
1626 F03
5.0 5.5 6.0 6.5 7.0 7.5
4.5
2.5
3.5 4.0
3.0
FIGURE 1 CIRCUIT
Inductor Core Selection
With the value of L selected, the type of inductor must be
chosen. Basically, there are two kinds of losses in an
inductor--core and copper losses.
Core losses are dependent on the peak-to-peak ripple
current and core material. However, they are independent
of the physical size of the core. By increasing inductance,
the peak-to-peak inductor ripple current will decrease,
therefore reducing core loss. Utilizing low core loss mate-
rial, such as molypermalloy or Kool M
will allow the user
to concentrate on reducing copper loss and preventing
saturation.
Although higher inductance reduces core loss, it increases
copper loss as it requires more windings. When space is
not a premium, larger wire can be used to reduce the wire
resistance. This also prevents excessive heat dissipation
in the inductor.
Catch Diode Selection
Losses in the catch diode depend on forward drop and
switching times. Therefore, Schottky diodes are a good
choice for low drop and fast switching times.
The catch diode carries the load current during the off-
time. The average diode current is therefore dependent on
Figure 3. Operating Frequency vs Supply Voltage
for Circuit Shown in Figure 1
8
LTC1626
ESR
COUT
< 2R
SENSE
To avoid overheating, the output capacitor must be sized
to handle the ripple current generated by the inductor. The
worst-case RMS ripple current in the output capacitor is
given by:
I
RMS
<
150mV/2R
SENSE
(A
RMS
)
Generally, once the ESR requirements for C
OUT
have been
met, the RMS current rating far exceeds the I
RIPPLE
requirement.
In some surface mount applications, multiple capacitors
may have to be paralleled to meet the capacitance, ESR or
RMS current handling requirement of the application.
Aluminum electrolyte and dry tantalum capacitors are
both available in surface mount configurations. In the case
of tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice is
the AVX TPS series of surface mount tantalums, available
in case heights ranging from 2mm to 4mm. Other capaci-
tor types include Sanyo OS-CON, Nichicon PL series and
Sprague 595D series. Consult the manufacturer for other
specific recommendations.
When the capacitance of C
OUT
is made too small, the
output ripple at low frequencies will be large enough to trip
the voltage comparator. This causes Burst Mode opera-
tion to be activated when the LTC1626 would normally be
in continuous mode operation. The effect will be most
pronounced with low R
SENSE
values and can be improved
at higher frequencies.
Low-Battery Detection
The low-battery detector senses the input voltage through
an external resistive divider. This divided voltage connects
to the () input of a voltage comparator (LBI) and is
compared to an internal 1.25V reference voltage. Neglect-
ing LBI
input bias current, the following expression is used
for setting the trip voltage threshold:
V
R
R
LB TRIP
_
.
=
+
1 25 1
4
3
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
The LBO
is an N-channel open drain that goes low when
the battery voltage drops below the threshold voltage. In
shutdown, the comparator is disabled and LBO
is in the
high impedance state. Figure 4 is a schematic diagram
detailing the low-battery comparator connection and op-
eration.
+
LBI
R4
1%
R3
1%
C
FILTER
0.01
F
V
IN
1.25V
LBO
1626 F04
LTC1626
Figure 4. Low-Battery Comparator
Setting the Output Voltage
The LTC1626 develops a 1.25V reference voltage between
the feedback pin V
FB
and the signal ground as shown in
Figure 5. By selecting resistor R1, a constant current is
caused to flow through R1 and R2 which sets the desired
output voltage. The regulated output voltage is deter-
mined by:
V
R
R
OUT
=
+
1 25 1
2
1
.
R1 should be
10k to ensure that sufficient current flows
through the divider to maintain accuracy and to provide a
minimum load for the regulator output at elevated
temperatures. (See Switch Leakage Current curve in Typi-
cal Performance Characteristics section.)
To prevent stray pickup, a 100pF capacitor is suggested
across R1, located close to the LTC1626.
100pF
R2
1%
R1
10k
1%
V
FB
SGND
V
OUT
1626 F05
LTC1626
Figure 5. Setting the Output Voltage
9
LTC1626
Thermal Considerations
In a majority of applications, the LTC1626 does not
dissipate much heat due to its high efficiency. However, in
applications where the switching regulator is running at
high duty cycles or the part is in dropout with the switch
turned on continuously (DC), some thermal analysis is
required. The goal of the thermal analysis is to determine
whether the power dissipated by the regulator exceeds the
maximum junction temperature. The temperature rise is
given by:
T
RISE
= P
D
JA
where P
D
is the power dissipated by the regulator and
JA
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature is given by:
T
J
= T
RISE
+ T
AMBIENT
As an example, consider the case when the LTC1626 is in
dropout at an input voltage of 3V with a load current of
0.5A. From the Typical Performance Characteristics graph
of Switch Resistance, the ON resistance of the P-channel
switch is 0.45
. Therefore, power dissipated by the
part is:
P
D
= I
2
R
DS(ON)
= 113mW
The SO package junction-to-ambient thermal resistance
JA
is 110
C/W. Therefore, the junction temperature of the
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
regulator when it is operating in a 25
C ambient tempera-
ture is:
T
J
= (0.113 110) + 25 = 38
C
Remembering that the above junction temperature is
obtained from an R
DS(ON)
at 25
C, we might recalculate
the junction temperature based on a higher R
DS(ON)
since
it increases with temperature. However, we can safely
assume that the actual junction temperature will not
exceed the absolute maximum junction temperature
of 125
C.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1626. These items are also illustrated graphically in
the layout diagram of Figure 6. Check the following in your
layout:
1. Are the signal and power grounds separated? The
LTC1626 signal ground (Pin 11) must return to the
() plate of C
OUT
. The power ground (Pin 12) returns
to the anode of the Schottky diode and the () plate
of C
IN
.
2. Does the (+) plate of C
IN
connect to the power V
IN
(Pins
1, 13) as close as possible? This capacitor provides the
AC current to the internal P-channel MOSFET and its
driver.
Figure 6. LTC1626 Layout Diagram (See Board Layout Checklist)
+
+
1
2
13
PWR V
IN
C
IN
C
OUT
R
SENSE
V
OUT
1626 F06
V
IN
BOLD LINES INDICATE
HIGH CURRENT PATHS
D1
0.1
F
L
R1
1k
C
T
1000pF
3900pF
R2
V
IN
SW
LTC1626
PWR V
IN
PGND
SGND
SHDN
V
FB
SENSE
+
SHUTDOWN
LBO
LBI
C
T
I
TH
SENSE
3
4
5
6
7
14
12
11
10
9
8
10
LTC1626
3. Is the input decoupling capacitor (0.1
F) connected
closely between power V
IN
(Pins 1, 13) and power
ground (Pin 12)? This capacitor carries the high fre-
quency peak currents.
4. Is the Schottky diode closely connected between the
power ground (Pin 12) and switch output (Pin 14)?
5. Does the LTC1626 SENSE
(Pin 7) connect to a point
close to R
SENSE
and the (+) plate of C
OUT
? The resistor
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
+
PWR V
IN
C
IN
47
F
16V
+
C
OUT
100
F
10V
FOR 3.3V:
R1 = 15k, 1%
R2 = 9.09k, 1%
R
SENSE
**
0.1
V
OUT
2.5V
0.25A
1626 TA02
D1
MBR0520LT1
L1*
22
H
R1
10k
1k
3- OR 4-CELL
NiCd OR NiMH
* SUMIDA CDRH62-220
** IRC 1206-R100F
AVX TPSD476K016
AVX TPSD107K010
3900pF
R2
10k
V
IN
(V
IN
= 2.7V TO 6V)
SW
LTC1626
PGND
SGND
SENSE
+
SENSE
SHDN
SHUTDOWN
LBO
I
TH
C
T
C
T
270pF
V
FB
LBI
+
0.1
F
1000pF
100pF
R4
R3
TYPICAL APPLICATIO
N
S
U
Single Cell Li-Ion to 2.5V Converter
+
PWR V
IN
C
IN
47
F
16V
+
C
OUT
100
F
10V
R
SENSE
**
0.1
V
OUT
2.5V
0.25A
1626 TA01
D1
MBR0520LT1
L1*
22
H
10k
1k
SINGLE
Li-ION
CELL
* SUMIDA CDRH62-220
** IRC 1206-R100F
AVX TPSD476K016
AVX TPSD107K010
3900pF
10k
V
IN
(V
IN
= 2.7V TO 4.5V)
SW
LTC1626
PGND
SGND
SENSE
+
SENSE
SHDN
SHUTDOWN
LBO
I
TH
C
T
C
T
270pF
V
FB
LBI
+
0.1
F
1000pF
100pF
divider R1-R2 must be connected between the (+) plate
of C
OUT
and the signal ground.
6. Are the SENSE
and SENSE
+
leads routed together with
minimum PC trace spacing? The 1000pF capacitor
between Pin 7 and Pin 8 should be as close as possible
to the LTC1626.
7. Is SHDN (Pin 10) actively pulled to ground during
normal operation? The shutdown pin is high imped-
ance and must not be allowed to float.
3- to 4-Cell NiCd/NiMH to 2.5V Converter
11
LTC1626
TYPICAL APPLICATIO
N
S
U
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
Low Profile (3mm Maximum Height) 2.8V Converter
+
PWR V
IN
C
IN
22
F
16V
TANT
+
C
OUT
100
F
6.3V
R
SENSE
**
0.1
V
OUT
2.8V
0.25A
1626 TA03
D1
MBR0520LT1
L1*
15
H
R1
15k
1%
1k
* COILCRAFT DO3308-153
** IRC 1206-R100F
AVX TPSC226M016R0375
AVX TPSC107M006R0150
MURATA GRM230Y5V475Z16
3900pF
R2
12.1k
1%
V
IN
V
IN
3V TO 6V
SW
LTC1626
PGND
SGND
SENSE
+
SENSE
SHDN
SHUTDOWN
LBO
I
TH
C
T
C
T
56pF
V
FB
LBI
4.7
F
CERAMIC
1000pF
100pF
Dimensions in inches (millimeters) unless otherwise noted.
PACKAGE DESCRIPTIO
N
U
S Package
14-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
1
2
3
4
0.150 0.157**
(3.810 3.988)
14
13
0.337 0.344*
(8.560 8.738)
0.228 0.244
(5.791 6.197)
12
11
10
9
5
6
7
8
0.016 0.050
0.406 1.270
0.010 0.020
(0.254 0.508)
45
0
8
TYP
0.008 0.010
(0.203 0.254)
S14 0695
0.053 0.069
(1.346 1.752)
0.014 0.019
(0.355 0.483)
0.004 0.010
(0.101 0.254)
0.050
(1.270)
TYP
DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
*
**
12
LTC1626
1626f LT/TP 0398 4K PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1997
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
q
(408) 432-1900
FAX: (408) 434-0507
q
TELEX: 499-3977
q
www.linear-tech.com
TYPICAL APPLICATIO
N
S
U
Single Li-Ion to 3.3V Buck-Boost Converter
PART NUMBER
DESCRIPTION
COMMENTS
LTC1174/LTC1174-3.3
High Efficiency Step-Down and Inverting DC/DC Converters
Monolithic Switching Regulators, Burst Mode Operation
LTC1174-5
LTC1265
1.2A, High Efficiency Step-Down DC/DC Converter
Constant Off-Time Monolithic, Burst Mode Operation
LT1375/LT1376
1.5A, 500kHz Step-Down Switching Regulators
High Frequency, Small Inductor, High Efficiency
LTC1435
High Efficiency, Low Noise, Synchronous Step-Down Converter
16-Pin Narrow SO and SSOP
LTC1436/LTC1436-PLL
High Efficiency, Low Noise, Synchronous Step-Down Converters 24-Pin Narrow and 28-Pin SSOP
LTC1438/LTC1439
Dual, Low Noise, Synchronous Step-Down Converters
Multiple Output Capability
LTC1474/LTC1475
Low Quiescent Current Step-Down DC/DC Converters
Monolithic, I
Q
= 10
A, 8-Pin MSOP
RELATED PARTS
+
+
PWR V
IN
C
IN
100
F
16V
100
F
16V
+
C
OUT
100
F
10V
R
SENSE
*
0.1
V
OUT
3.3V
1626 TA05
D1
MBRS130LT1
L1A
33
H
15k
1%
1k
SINGLE
Li-ION
CELL
* IRC 1206-R100F
AVX TPSE107M016R0100
AVX TPSD107M010R0065
3900pF
9.09k
1%
V
IN
(V
IN
= 2.5V TO 4.2V)
SW
1
2
L1B
33
H
4
3
LTC1626
PGND
SGND
SENSE
SENSE
+
SHDN
SHUTDOWN
LBO
I
TH
C
T
C
T
75pF
V
FB
LBI
+
0.1
F
1000pF
100pF
L1A
L1A
4
3
2
TOP VIEW
1
L1B
L1B
MANUFACTURER
PART NO.
COILTRONICS
CTX33-4
DALE
LPT4545-330LA
V
IN
(V)
I
OUT
(mA)
2.5
200
3.0
350
3.5
500*
4.0
500*
4.2
500*
*DESIGN LIMIT
+
PWR V
IN
C
IN
100
F
10V
+
C
OUT
220
F
10V
R
SENSE
**
0.1
1626 TA04
D1
MBRS130LT1
L1*
47
H
15k
1%
1k
* COILCRAFT DO3316-473
** IRC 1206-R100F
AVX TPSD107K010
AVX TPSE227K010
3900pF
9.09k
1%
V
IN
V
IN
5V
SW
LTC1626
PGND
SGND
SENSE
+
SENSE
SHDN
SHUTDOWN
LBO
I
TH
C
T
C
T
270pF
V
FB
LBI
0.1
F
1000pF
100pF
V
OUT
3.3V
0.5A
5V to 3.3V Converter
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