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1
LT1507
500kHz Monolithic
Buck Mode Switching Regulator
5V to 3.3V Efficiency
LOAD CURRENT (A)
0
EFFICIENCY (%)
100
90
80
70
60
50
1.00
LT1507 TA02
0.25
0.50
0.75
1.25
V
IN
= 5V
V
OUT
= 3.3V
TYPICAL APPLICATIO
N
U
5V to 3.3V Volt Down Converter
V
IN
SHDN
V
SW
SENSE
BOOST
GND
V
C
LT1507-3.3
D2
1N914
C2
0.1
F
L1***
5
H
D1
1N5818
C
C
3.3nF
+
C1**
100
F
10V
TANTALUM
OUTPUT
3.3V
1.25A
DEFAULT
(OPEN)
= ON
+
C3*
47
F
16V
TANTALUM
5V
*
**
***
AVX TPSD477M016R0150 OR SPRAGUE 593D EQUIVALENT.
RIPPLE CURRENT RATING
0.6A
AVX TPSD108M010R0100 OR SPRAGUE 593D EQUIVALENT
COILTRONICS CTX5-1. SUBSTITUTION UNITS SHOULD BE RATED
AT
1.25A, USING LOW LOSS CORE MATERIAL
SEE BOOST PIN CONSIDERATIONS IN APPLICATIONS INFORMATION
SECTION FOR ALTERNATIVE D2 CONNECTION
with all the necessary oscillator, control and logic cir-
cuitry. High switching frequency allows a considerable
reduction in the size of external components. The topology
is current mode for fast transient response and good loop
stability. Both fixed output voltage (3.3V) and adjustable
parts are available.
A special high speed bipolar process and new design
techniques allow this regulator to achieve high efficiency
at a high switching frequency. Efficiency is maintained
over a wide output current range by keeping quiescent
supply current to 4mA and by utilizing a supply boost
capacitor to allow the NPN power switch to saturate. A
shutdown signal will reduce supply current to 20
A. The
LT1507 can be externally synchronized from 570kHz to
1MHz with logic level inputs.
The LT1507 fits into standard 8-pin SO and PDIP pack-
ages. Temperature rise is kept to a minimum by the high
efficiency design. Full cycle-by-cycle short-circuit protec-
tion and thermal shutdown are provided. Standard surface
mount external parts are used including the inductor and
capacitors.
, LTC and LT are registered trademarks of Linear Technology Corporation.
s
Portable Computers
s
Battery-Powered Systems
s
Battery Charger
s
Distributed Power
APPLICATIO
N
S
U
FEATURES
s
Constant 500kHz Switching Frequency
s
Uses All Surface Mount Components
s
Operates with Inputs as Low as 4V
s
Saturated Switch Design (0.3
)
s
Cycle-by-Cycle Current Limiting
s
Easily Synchronizable
s
Inductor Size as Low as 2
H
s
Shutdown Current: 20
A
The LT
1507 is a 500kHz monolithic buck mode switching
regulator, functionally identical to the LT1375 but opti-
mized for lower input voltage applications. It will operate
over a 4V to 15V input range, compared with 5.5V to 25V
for the LT1375. A 1.5A switch is included on the die along
DESCRIPTIO
N
U
2
LT1507
Input Voltage ........................................................... 16V
Boost Pin Voltage .................................................... 25V
Shutdown Pin Voltage ............................................... 7V
FB Pin Voltage (Adjustable Part) ............................. 3.5V
FB Pin Current (Adjustable Part) ............................. 1mA
Sense Voltage (Fixed 3.3V Part) ................................ 5V
Sync Pin Voltage ....................................................... 7V
Operating Ambient Temperature Range
LT1507C .................................................. 0
C to 70
C
LT1507I .............................................. 40
C to 85
C
Max Operating Junction Temperature ................... 125
C
Storage Temperature Range ................. 65
C to 150
C
Lead Temperature (Soldering, 10 sec).................. 300
C
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
T
JMAX
= 125
C,
JA
= 80
C/W TO 120
C/ W (N)
T
JMAX
= 125
C,
JA
= 120
C/W TO 170
C/ W (S)
DEPENDING ON PC BOARD LAYOUT
1
2
3
4
8
7
6
5
TOP VIEW
V
C
FB/SENSE
GND
SYNC
BOOST
V
IN
V
SW
SHDN
N8 PACKAGE
8-LEAD PDIP
S8 PACKAGE
8-LEAD PLASTIC SO
Consult factory for Military grade parts.
ORDER PART
NUMBER
LT1507CN8
LT1507CN8-3.3
LT1507CS8
LT1507CS8-3.3
LT1507IN8
LT1507IN8-3.3
LT1507IS8
LT1507IS8-3.3
S8 PART MARKING
ELECTRICAL CHARACTERISTICS
T
J
= 25
C, V
IN
= 5V, V
C
= 1.5V, boost open, switch open unless otherwise specified.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Reference Voltage (Adjustable)
2.39
2.42
2.45
V
All Conditions
q
2.36
2.48
V
Sense Voltage (3.3V)
3.25
3.3
3.35
V
All Conditions
q
3.23
3.37
V
Sense Pin Resistance
q
4.0
6.6
9.5
k
Reference Voltage Line Regulation
4.3V
V
IN
15V
q
0.01
0.03
%/V
FB Input Bias Current
q
0.5
2
A
Error Amplifier Voltage Gain (Note 8)
(Note 1)
150
400
Error Amplifier Transconductance (Note 8)
I(V
C
) =
10
A
1500
2000
2700
mho
q
1100
3000
mho
V
C
Pin to Switch Current
Transconductance
2
A/V
Error Amplifier Source Current
V
FB
= 2.1V or V
SENSE
= 2.9V
q
150
225
320
A
Error Amplifier Sink Current
V
FB
= 2.7V or V
SENSE
= 3.7V
2
mA
V
C
Pin Switching Threshold
Duty Cycle = 0
0.9
V
V
C
Pin High Clamp
V
FB
= 2.1V or V
SENSE
= 2.9V
2.1
V
Switch Current Limit
V
C
Open, V
FB
= 2.1V or V
SENSE
= 2.9V
DC
50%
q
1.50
2
3
A
V
IN
5V, V
BOOST
= V
IN
+ 5V
DC = 80%
q
1.35
3
A
Switch On Resistance (Note 6)
I
SW
= 1.5A, V
BOOST
= V
IN
+ 5V
0.3
0.4
q
0.5
Maximum Switch Duty Cycle
V
FB
= 2.1V or V
SENSE
= 2.9V
90
93
%
q
86
93
%
1507I
1507I3
1507
15073
3
LT1507
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Switch Frequency
V
C
Set to Give 50% Duty Cycle
460
500
540
kHz
25
C
T
J
125
C
440
560
kHz
T
J
25
C
440
570
kHz
Switch Frequency Line Regulation
4.3V
V
IN
15V
q
0.05
0.15
%/V
Frequency Shifting Threshold on F
B
Pin
f = 10kHz
q
0.8
1.0
1.3
V
Minimum Input Voltage (Note 2)
q
4
4.3
V
Minimum Boost Voltage (Note 3)
I
SW
1.5A
q
3
3.5
V
Boost Current (Note 4)
V
BOOST
= V
IN
+ 5V
I
SW
= 500mA, 25
C
T
J
125
C
12
22
mA
T
J
25
C
25
mA
I
SW
= 1.5A,
25
C
T
J
125
C
25
35
mA
T
J
25
C
40
mA
Input Supply Current (Note 5)
q
3.8
5.4
mA
Shutdown Supply Current
V
SHDN
= 0V, V
IN
12V
15
50
A
V
SW
= 0V, V
C
Open
q
75
A
Lockout Threshold
V
C
Open
q
2.3
2.38
2.46
V
Shutdown Threshold
V
C
Open
Device Shutting Down
q
0.15
0.37
0.70
V
Device Starting Up
q
0.25
0.45
0.70
V
Minimum Synchronizing Amplitude
q
1.5
2.2
V
Synchronizing Frequency Range (Note 7)
580
1000
kHz
The
q
denotes specifications which apply over the operating temperature
range.
Note 1: Gain is measured with a V
C
swing equal to 200mV above the low
clamp level to 200mV below the upper clamp level.
Note 2: Minimum input voltage is not measured directly, but is guaranteed
by other tests. It is defined as the voltage where internal bias lines are still
regulated, so that the reference voltage and oscillator frequency remain
constant. Actual minimum input voltage to maintain a regulated output will
depend on output voltage and load current. See Applications Information.
Note 3: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the internal power switch.
Note 4: Boost current is the current flowing into the BOOST pin with the
pin held 5V above input voltage. It flows only during switch ON time.
Note 5: Input supply current is the bias current drawn by the V
IN
pin when
the SHDN pin is held at 1V (switching disabled).
Note 6: Switch ON resistance is calculated by dividing V
IN
to V
SW
voltage
by the forced current (1.5A). See Typical Performance Characteristics for
the graph of switch voltage at other currents.
Note 7: For synchronizing frequency above 700kHz, with duty cycles
above 50%, external slope compensation may be needed. See Applications
Information.
Note 8: Transconductance and voltage gain refer to the internal amplifier
exclusive of the voltage divider. To calculate gain and transconductance
refer to SENSE pin on fixed voltage parts. Divide values shown by the ratio
V
OUT
/2.42.
T
J
= 25
C, V
IN
= 5V, V
C
= 1.5V, boost open, switch open unless otherwise specified.
V
C
Pin Shutdown Threshold
JUNCTION TEMPERATURE (
C)
50
2.44
2.43
2.42
2.41
2.40
100
LT1507 TPC03
25
0
25
50
75
125
FEEDBACK VOLTAGE (V)
CURRENT (
A)
2.0
1.5
1.0
0.5
0
VOLTAGE
CURRENT
LOAD CURRENT (A)
0
EFFICIENCY (%)
100
90
80
70
60
50
1.00
LT1507 TA02
0.25
0.50
0.75
1.25
V
IN
= 5V
V
OUT
= 3.3V
Switch Peak Current Limit
Feedback Pin Voltage and Current
JUNCTION TEMPERATURE (
C)
50
1.4
1.2
1.0
0.8
0.6
0.4
100
LT1507 TPC01
25
0
25
50
75
125
THRESHOLD VOLTAGE (V)
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
4
LT1507
Shutdown Pin Bias Current
JUNCTION TEMPERATURE (
C)
50
2.40
2.36
2.32
0.8
0.4
0
25
75
LT1507 TPC05
25
0
50
100
125
SHUTDOWN PIN VOLTAGE (V)
STANDBY
STARTUP
SHUTDOWN
Standby and Shutdown Thresholds
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
INPUT VOLTAGE (V)
0
0
INPUT SUPPLY CURRENT (
A)
5
10
15
20
25
30
3
6
9
12
LT1507 TPC06
15
V
SHDN
= 0V
Shutdown Supply Current
Shutdown Supply Current
Error Amplifier Transconductance
TEMPERATURE (
C)
50
500
400
300
200
8
4
0
25
75
L11507 TPC04
25
0
50
100
125
CURRENT (
A)
CURRENT REQUIRED TO FORCE SHUTDOWN
(FLOWS OUT OF PIN). AFTER SHUTDOWN,
CURRENT DROPS TO A FEW
A
AT 2.38V STANDBY THRESHOLD
(CURRENT FLOWS OUT OF PIN)
JUNCTION TEMPERATURE (
C)
50
TRANSCONDUCTANCE (
mho)
2500
2000
1500
1000
500
0
0
50
75
LT1507 TPC08
25
25
100
125
SHUTDOWN VOLTAGE (V)
0
INPUT SUPPLY CURRENT (
A)
150
125
100
75
50
25
0
0.1
0.2
0.3
0.4
LT1507 TPC07
0.5
V
IN
= 10V
FREQUENCY (Hz)
GAIN (
mho)
PHASE (DEG)
3000
2500
2000
1500
1000
500
200
150
100
50
0
50
100
10k
100k
10M
LT1507 TPC09
1k
1M
GAIN
PHASE
R
OUT
200k
C
OUT
12pF
V
C
V
FB
2e
3
ERROR AMPLIFIER EQUIVALENT CIRCUIT
R
LOAD
= 50
Error Amplifier Transconductance
Minimum Input Voltage
with 3.3V Output
Switching Frequency
FEEDBACK PIN VOLTAGE (V)
0
SWITCHING FREQUENCY (kHz) OR CURRENT (
A) 500
400
300
200
100
0
2.0
LT1507 TPC10
0.5
1.0
1.5
2.5
SWITCHING
FREQUENCY
FEEDBACK PIN
CURRENT
Frequency Foldback
JUNCTION TEMPERATURE (
C)
50
600
550
500
450
400
100
LT1507 TPC11
25
0
25
50
75
125
FREQUENCY (kHz)
LOAD CURRENT (mA)
1
5.0
INPUT VOLTAGE (V)
5.5
6.0
6.5
10
100
1000
LT1507 TPC12
4.5
4.0
3.5
3.0
MINIMUM VOLTAGE
TO START WITH
STANDARD CIRCUIT
MINIMUM VOLTAGE
TO RUN WITH
STANDARD CIRCUIT
MINIMUM INPUT VOLTAGE CAN BE REDUCED
BY ADDING A SMALL EXTERNAL PNP. SEE
APPLICATIONS INFORMATION
5
LT1507
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
Inductor Core Loss for 3.3V Output
INDUCTANCE (
H)
1
0.001
CORE LOSS (W)
0.1
0.01
1.0
2
4
6
8
10
LT1507 TPC17
TYPE 52 POWDERED IRON
Kool M
PERMALLOY
= 125
Metglas
CORE LOSS IS INDEPENDENT OF LOAD CURRENT
UNTIL LOAD CURRENT FALLS LOW ENOUGH
FOR CIRCUIT TO GO INTO DISCONTINUOUS MODE
V
OUT
= 3.3V
V
IN
= 5V
I
OUT
= 1A
Kool M
is a registered trademark of Magnetics, Incorporated.
Metglas is a registered trademark of AlliedSignal Incorporated.
BOOST (Pin 1): The BOOST pin is used to provide a drive
voltage, higher than the input voltage, to the internal
bipolar NPN power switch. Without this added voltage the
typical switch voltage loss would be about 1.5V. The
additional boost voltage allows the switch to saturate and
voltage loss approximates that of a 0.3
FET structure,
but with a much smaller die area. Efficiency improves from
PI
N
FU
N
CTIO
N
S
U
U
U
70% for conventional bipolar designs to greater than 85%
for these new parts.
V
IN
(Pin 2): Input Pin. The LT1507 is designed to operate
with an input voltage between 4.5V and 15V. Under certain
conditions, input voltage may be reduced down to 4V.
Actual minimum operating voltage will always be higher
than the output voltage. It may be limited by switch
OUTPUT VOLTAGE (%)
0
OUTPUT CURRENT (A)
2.5
2.0
1.5
1.0
0.5
0
80
LT1507 TPC13
20
40
60
100
MOS LOAD
RESISTOR LOAD
CURRENT
SOURCE LOAD
FOLDBACK
CHARACTERISTICS
*POSSIBLE
UNDESIRED
STABLE POINT
FOR CURRENT
SOURCE LOAD
*SEE "MORE THAN JUST VOLTAGE FEEDBACK"
IN APPLICATIONS INFORMATION SECTION
Current Limit Foldback
INPUT VOLTAGE (V)
4
0
CURRENT (A)
0.25
0.50
0.75
1.00
1.25
1.50
6
8
10
12
LT1507 TPC14
14
L = 10
H
L = 5
H
L = 3
H
L = 2
H
V
OUT
= 3.3V
Maximum Load Current
at V
OUT
= 3.3V
INPUT VOLTAGE (V)
0
0
CURRENT (A)
0.25
0.50
0.75
1.00
1.25
1.50
3
6
9
12
LT1507 TPC15
15
L = 10
H
L = 5
H
L = 20
H
Maximum Load Current
at V
OUT
= 5V
Boost Pin Current
SWITCH CURRENT (A)
0
BOOST PIN CURRENT (mA)
12
10
8
6
4
2
0
0.25
0.50
0.75
1.00
LT1507 TPC16
1.25
T
J
= 25
C
Switch Voltage Drop
SWITCH CURRENT (A)
0
SWITCH VOLTAGE (V)
0.8
0.6
0.4
0.2
0
0.25
0.50
0.75
1.00
LT1507 TPC18
1.25
1.50
T
J
= 25
C
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