1
LT1959
DESCRIPTIO
U
FEATURES
APPLICATIO S
U
TYPICAL APPLICATIO
N
U
4.5A, 500kHz Step-Down
Switching Regulator
s
Operates with Input as Low as 4V
s
Output Range Down to 1.21V
s
Constant 500kHz Switching Frequency
s
Uses All Surface Mount Components
s
Inductor Size Reduced to 1.8
H
s
Saturating Switch Design: 0.07
s
Shutdown Current: 20
A
s
Easily Synchronizable
s
Cycle-by-Cycle Current Limiting
s
4.5A Switch
s
Current Mode Control
The LT
1959 is a 500kHz monolithic buck mode switching
regulator functionally identical to the LT1506 but optimized
for lower output voltage applications. It will operate down
to 1.21V output compared to 2.42V for the LT1506. A 4.5A
switch is included on the die along with all the necessary
oscillator, control and logic circuitry. High switching fre-
quency allows a considerable reduction in the size of ex-
ternal components. The topology is current mode for fast
transient response and good loop stability.
A special high speed bipolar process and new design tech-
niques achieve high efficiency at high switching frequency.
Efficiency is maintained over a wide output current range
by keeping quiescent supply current to 4mA and by utiliz-
ing a supply boost capacitor to saturate the power switch.
The LT1959 fits into standard 7-pin DD and fused lead
SO-8 packages. Full cycle-by-cycle short-circuit protection
and thermal shutdown are provided. Standard surface
mount external parts are used, including the inductor and
capacitors. There is the optional function of shutdown or
synchronization. A shutdown signal reduces supply current
to 20
A. Synchronization allows an external logic level sig-
nal to increase the internal oscillator from 580kHz to 1MHz.
s
Portable Computers
s
Battery-Powered Systems
s
Battery Chargers
s
Distributed Power
s
5V to 3.3V Conversion
s
5V to 2.5V Conversion
s
5V to 1.8V Conversion
, LTC and LT are registered trademarks of Linear Technology Corporation.
Efficiency vs Load Current
5V to 1.8V Down Converter
BOOST
LT1959
V
IN
OUTPUT
1.8V
4A
INPUT
5V
1959 TA01
C2
0.68
F
C
C
1.5nF
D1
MBRS330T3
C1
100
F, 10V
SOLID
TANTALUM
C3
10
F TO
50
F
CERAMIC
D2
1N914
L1
5
H
V
SW
FB
SHDN
OPEN
OR
HIGH
= ON
GND
V
C
+
+
R2
2.49k
R1
1.21k
LOAD CURRENT (A)
0
EFFICIENCY (%)
90
85
80
75
70
2.0
2.5
3.0
3.5
1959 TA02
0.5
1.0
1.5
4.0
V
OUT
= 3.3V
V
IN
= 5V
L = 10
H
2
LT1959
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C, T
J
= 25
C, V
IN
= 5V, V
C
= 1.5V, Boost = V
IN
+ 5V, switch open, unless
otherwise noted.
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
Input Voltage .......................................................... 16V
BOOST Voltage ........................................................ 30V
BOOST Pin Above Input Voltage ............................. 15V
SHDN Pin Voltage ..................................................... 7V
FB Pin Voltage ....................................................... 3.5V
FB Pin Current ....................................................... 1mA
SYNC Pin Voltage ..................................................... 7V
Operating Junction Temperature Range
LT1959C ................................................ 0
C to 125
C
LT1959I ........................................... 40
C to 125
C
Storage Temperature Range ................ 65
C to 150
C
Lead Temperature (Soldering, 10 sec)................. 300
C
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Feedback Voltage (Adjustable)
All Conditions
q
1.19
1.21
1.23
V
Reference Voltage Line Regulation
4.3V
V
IN
15V
0.01
0.03
%/ V
Feedback Input Bias Current
q
0.5
0
0.5
A
Error Amplifier Voltage Gain
(Note 2)
200
400
Error Amplifier Transconductance
I (V
C
) =
10
A
1500
2000
2700
Mho
q
1000
3100
Mho
V
C
Pin to Switch Current Transconductance
5.3
A/ V
Error Amplifier Source Current
V
FB
= 1.05V
q
140
225
320
A
Error Amplifier Sink Current
V
FB
= 1.35V
q
140
225
320
A
V
C
Pin Switching Threshold
Duty Cycle = 0
0.9
V
V
C
Pin High Clamp
2.1
V
Switch Current Limit
V
C
Open, V
FB
= 1.05V, DC
50%
q
4.5
6
8.5
A
Slope Compensation
DC = 80%
0.8
A
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
ORDER PART
NUMBER
ORDER PART
NUMBER
LT1959CR
LT1959IR
1959
1959I
S8 PART MARKING
LT1959CS8
LT1959IS8
(Note 1)
1
2
3
4
8
7
6
5
TOP VIEW
S8 PACKAGE
8-LEAD PLASTIC SO
V
IN
BOOST
GND**
V
SW
SYNC
SHDN
V
C
FB
R PACKAGE
7-LEAD PLASTIC DD
FRONT VIEW
FB
BOOST
V
IN
GND
V
SW
7
6
5
4
3
2
1
SHDN
V
C
TAB
IS
GND
**WITH (FUSED GND) GROUND PIN
CONNECTED TO GROUND PLANE OR
LARGE LANDS
T
JMAX
= 150
C,
JA
= 30
C/W
WITH PACKAGE SOLDERED TO 0.5 SQUARE INCH
COPPER AREA OVER BACKSIDE GROUND PLANE OR
INTERNAL POWER PLANE.
JA
CAN VARY FROM 20
C/W
TO > 40
C/W DEPENDING ON MOUNTING TECHNIQUES
T
JMAX
= 150
C,
JA
= 80
C/W
3
LT1959
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Switch On Resistance (Note 7)
I
SW
= 4.5A
0.07
0.1
q
0.13
Maximum Switch Duty Cycle
V
FB
= 1.05V
90
93
%
q
86
93
%
Switch Frequency
V
C
Set to Give 50% Duty Cycle
460
500
540
kHz
q
440
560
kHz
Switch Frequency Line Regulation
4.3V
V
IN
15V
q
0
0.15
%/ V
Frequency Shifting Threshold on FB Pin
f = 10kHz
q
0.5
0.7
1.0
V
Minimum Input Voltage (Note 3)
q
4.0
4.3
V
Minimum Boost Voltage (Note 4)
I
SW
4.5A
q
2.3
3.0
V
Boost Current (Note 5)
I
SW
= 1A
q
20
35
mA
I
SW
= 4.5A
q
90
140
mA
Input Supply Current (Note 6)
q
3.8
5.4
mA
Shutdown Supply Current
V
SHDN
= 0V, V
SW
= 0V, V
C
Open
15
50
A
q
75
A
Lockout Threshold
V
C
Open
q
2.3
2.38
2.46
V
Shutdown Thresholds
V
C
Open Device Shutting Down
q
0.13
0.37
0.60
V
Device Starting Up
q
0.25
0.45
0.7
V
Synchronization Threshold
q
1.5
2.2
V
Synchronizing Range
580
1000
kHz
SYNC Pin Input Resistance
40
k
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: Gain is measured with a V
C
swing equal to 200mV above the
switching threshold level to 200mV below the upper clamp level.
Note 3: 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 4: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the internal power switch.
Note 5: 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 6: Input supply current is the bias current drawn by the input pin
with switching disabled.
Note 7: Switch on resistance is calculated by dividing V
IN
to V
SW
voltage
by the forced current (4.5A). See Typical Performance Characteristics for
the graph of switch voltage at other currents.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C, T
J
= 25
C, V
IN
= 5V, V
C
= 1.5V, Boost = V
IN
+ 5V, switch open, unless
otherwise noted.
4
LT1959
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
Feedback Pin Voltage
Switch Peak Current Limit
Lockout and Shutdown
Thresholds
Shutdown Pin Bias Current
Shutdown Supply Current
Shutdown Supply Current
Error Amplifier Transconductance
Error Amplifier Transconductance
Minimum Input Voltage
with 3.3V Output
LOAD CURRENT (mA)
1
4.1
INPUT VOLTAGE (V)
4.3
4.5
4.7
10
100
1000
1959 G01
3.9
3.7
3.5
3.3
DUTY CYCLE (%)
0
SWITCH PEAK CURRENT (A)
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
80
1959 G02
20
40
60
100
TYPICAL
MINIMUM
TEMPERATURE (
C)
50
1.215
1.210
1.205
100
1959 G03
25
0
25
50
75
125
FEEDBACK VOLTAGE (V)
TEMPERATURE (
C)
50
500
400
300
200
8
4
0
25
75
1959 G04
25
0
50
100
125
CURRENT (
A)
AT 0.37V SHUTDOWN THRESHOLD.
AFTER SHUTDOWN, CURRENT
DROPS TO A FEW
A
AT 2.38V LOCKOUT THRESHOLD
JUNCTION TEMPERATURE (
C)
50
2.40
2.36
2.32
0.8
0.4
0
25
75
1959 G05
25
0
50
100
125
SHUTDOWN PIN VOLTAGE (V)
LOCKOUT
START-UP
SHUTDOWN
INPUT VOLTAGE (V)
0
INPUT SUPPLY CURRENT (
A)
25
20
15
10
5
0
5
10
15
1959 G06
V
SHDN
= 0V
FREQUENCY (Hz)
GAIN (
Mho)
PHASE (DEG)
3000
2500
2000
1500
1000
500
200
150
100
50
0
50
100
10k
100k
10M
1959 G09
1k
1M
GAIN
PHASE
ERROR AMPLIFIER EQUIVALENT CIRCUIT
R
OUT
200k
C
OUT
12pF
V
C
R
LOAD
= 50
V
FB 2
10
3
)
(
JUNCTION TEMPERATURE (
C)
50
TRANSCONDUCTANCE (
Mho)
2500
2000
1500
1000
500
0
0
50
75
1959 G08
25
25
100
125
SHUTDOWN VOLTAGE (V)
0
INPUT SUPPLY CURRENT (
A)
70
60
50
40
30
20
10
0
0.1
0.2
0.3
0.4
1959 G07
V
IN
= 10V
5
LT1959
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
Switching Frequency
BOOST Pin Current
Maximum Load Current
at V
OUT
= 3.3V
Maximum Load Current
at V
OUT
= 5V
Switch Voltage Drop
V
C
Pin Shutdown Threshold
Current Limit Foldback
*See "More Than Just Voltage Feedback" in the Applications Information section.
TEMPERATURE (
C)
50
550
540
530
520
510
500
490
480
470
460
450
100
1959 G11
25
0
25
50
75
125
FREQUENCY (kHz)
INPUT VOLTAGE (V)
5
2.6
LOAD CURRENT (A)
2.8
3.2
3.4
3.6
9
13
15
4.4
1959 G13
3.0
7
11
3.8
4.0
4.2
L= 10
H
L= 5
H
L= 3
H
L= 1.8
H
SWITCH CURRENT (A)
0
0
BOOST PIN CURRENT (mA)
10
20
40
30
60
50
1
2
3
4
5
1959 G15
80
70
100
90
DUTY CYCLE = 100%
INPUT VOLTAGE (V)
4
4.0
4.2
4.4
12
1959 G14
3.8
3.6
6
10
8
14
3.4
3.2
3.0
LOAD CURRENT (A)
L= 10
H
L= 5
H
L= 3
H
L= 1.8
H
OUTPUT VOLTAGE (%)
0
5
6
7
80
1959 G16
4
3
20
40
60
100
2
1
0
OUTPUT CURRENT (A)
FOLDBACK
CHARACTERISTICS
CURRENT
SOURCE
LOAD
RESISTOR
LOAD
MOS LOAD
POSSIBLE UNDESIRED
STABLE POINT FOR
CURRENT SOURCE
LOAD*
JUNCTION TEMPERATURE (
C)
50
1.4
1.2
1.0
0.8
0.6
0.4
100
1959 G17
25
0
25
50
75
125
THRESHOLD VOLTAGE (V)
SHUTDOWN
SWITCH CURRENT (A)
0
SWITCH VOLTAGE (mV)
250
200
400
125
C
25
C
350
300
4
5
1959 G18
150
100
50
0
1
2
3
500
450
40
C
Frequency Foldback
FEEDBACK PIN VOLTAGE (V)
0
SWITCHING FREQUENCY (kHz) OR CURRENT (
A) 500
400
300
200
100
0
0.2
0.4
0.6
0.8
1959 G10
1.0
1.2
SWITCHING
FREQUENCY
FEEDBACK
PIN CURRENT
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