LT1959
4.5A, 500kHz Step-Down
Switching Regulator
FEATURES
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DESCRIPTIO
Operates with Input as Low as 4V
Output Range Down to 1.21V
Constant 500kHz Switching Frequency
Uses All Surface Mount Components
Inductor Size Reduced to 1.8
µ
H
Saturating Switch Design: 0.07Ω
Shutdown Current: 20µA
Easily Synchronizable
Cycle-by-Cycle Current Limiting
4.5A Switch
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 20µA. Synchronization allows an external logic level sig-
nal to increase the internal oscillator from 580kHz to 1MHz.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
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Portable Computers
Battery-Powered Systems
Battery Chargers
Distributed Power
5V to 3.3V Conversion
5V to 2.5V Conversion
5V to 1.8V Conversion
TYPICAL APPLICATION
5V to 1.8V Down Converter
D2
1N914
C2
0.68µF
INPUT
5V
BOOST
V
IN
V
SW
LT1959
SHDN
GND
FB
V
C
C
C
1.5nF
D1
MBRS330T3
R2
2.49k
R1
1.21k
85
L1
5µH
90
C3
10µF TO
50µF
CERAMIC
+
OPEN
OR
HIGH
= ON
OUTPUT
1.8V
4A
EFFICIENCY (%)
80
+
C1
100µF, 10V
SOLID
TANTALUM
1959 TA01
75
70
0
0.5
1.0
1.5 2.0 2.5 3.0
LOAD CURRENT (A)
3.5
4.0
U
Efficiency vs Load Current
V
OUT
= 3.3V
V
IN
= 5V
L = 10µH
1959 TA02
U
U
1
LT1959
ABSOLUTE
MAXIMUM
RATINGS
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
PACKAGE/ORDER INFORMATION
FRONT VIEW
7
6
5
4
3
2
1
R PACKAGE
7-LEAD PLASTIC DD
FB
BOOST
V
IN
GND
V
SW
SHDN
V
C
ORDER PART
NUMBER
LT1959CR
LT1959IR
V
IN
1
BOOST 2
FB 3
GND** 4
TAB
IS
GND
T
JMAX
= 150°C,
θ
JA
= 30°C/W
WITH PACKAGE SOLDERED TO 0.5 SQUARE INCH
COPPER AREA OVER BACKSIDE GROUND PLANE OR
INTERNAL POWER PLANE.
θ
JA
CAN VARY FROM 20°C/W
TO > 40°C/W DEPENDING ON MOUNTING TECHNIQUES
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.
PARAMETER
Feedback Voltage (Adjustable)
Reference Voltage Line Regulation
Feedback Input Bias Current
Error Amplifier Voltage Gain
Error Amplifier Transconductance
V
C
Pin to Switch Current Transconductance
Error Amplifier Source Current
Error Amplifier Sink Current
V
C
Pin Switching Threshold
V
C
Pin High Clamp
Switch Current Limit
Slope Compensation
CONDITIONS
All Conditions
4.3V
≤
V
IN
≤
15V
(Note 2)
∆I
(V
C
) =
±10µA
q
ELECTRICAL CHARACTERISTICS
V
FB
= 1.05V
V
FB
= 1.35V
Duty Cycle = 0
V
C
Open, V
FB
= 1.05V, DC
≤
50%
DC = 80%
2
U
U
W
W W
U
W
(Note 1)
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
TOP VIEW
8 V
SW
7 SYNC
6 SHDN
5 V
C
ORDER PART
NUMBER
LT1959CS8
LT1959IS8
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 150°C,
θ
JA
= 80°C/W
**WITH (FUSED GND) GROUND PIN
CONNECTED TO GROUND PLANE OR
LARGE LANDS
S8 PART MARKING
1959
1959I
q
q
MIN
1.19
–0.5
200
1500
1000
140
140
TYP
1.21
0.01
0
400
2000
5.3
225
225
0.9
2.1
6
0.8
MAX
1.23
0.03
0.5
2700
3100
320
320
UNITS
V
%/ V
µA
µMho
µMho
A/ V
µA
µA
V
V
A
A
q
q
q
4.5
8.5
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.
PARAMETER
Switch On Resistance (Note 7)
Maximum Switch Duty Cycle
Switch Frequency
Switch Frequency Line Regulation
Frequency Shifting Threshold on FB Pin
Minimum Input Voltage (Note 3)
Minimum Boost Voltage (Note 4)
Boost Current (Note 5)
Input Supply Current (Note 6)
Shutdown Supply Current
Lockout Threshold
Shutdown Thresholds
Synchronization Threshold
Synchronizing Range
SYNC Pin Input Resistance
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.
CONDITIONS
I
SW
= 4.5A
q
ELECTRICAL CHARACTERISTICS
MIN
TYP
0.07
93
93
500
0
0.7
4.0
2.3
20
90
3.8
15
2.38
0.37
0.45
1.5
40
MAX
0.1
0.13
V
FB
= 1.05V
q
V
C
Set to Give 50% Duty Cycle
q
90
86
460
440
0.5
4.3V
≤
V
IN
≤
15V
∆f
= 10kHz
I
SW
≤
4.5A
I
SW
= 1A
I
SW
= 4.5A
V
SHDN
= 0V, V
SW
= 0V, V
C
Open
q
q
q
q
q
q
q
q
V
C
Open
V
C
Open Device Shutting Down
Device Starting Up
q
q
q
q
2.3
0.13
0.25
580
540
560
0.15
1.0
4.3
3.0
35
140
5.4
50
75
2.46
0.60
0.7
2.2
1000
UNITS
Ω
Ω
%
%
kHz
kHz
%/ V
V
V
V
mA
mA
mA
µA
µA
V
V
V
V
kHz
kΩ
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.
3
LT1959
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Input Voltage
with 3.3V Output
4.7
4.5
SWITCH PEAK CURRENT (A)
INPUT VOLTAGE (V)
4.3
4.1
3.9
3.7
3.5
3.3
1
10
100
LOAD CURRENT (mA)
1000
1959 G01
5.5
5.0
MINIMUM
4.5
4.0
3.5
3.0
0
20
60
40
DUTY CYCLE (%)
80
100
1959 G02
FEEDBACK VOLTAGE (V)
Shutdown Pin Bias Current
–500
– 400
– 300
– 200
–8
AT 2.38V LOCKOUT THRESHOLD
–4
0
–50 –25
AT 0.37V SHUTDOWN THRESHOLD.
AFTER SHUTDOWN, CURRENT
DROPS TO A FEW
µA
2.40
2.36
INPUT SUPPLY CURRENT (µA)
SHUTDOWN PIN VOLTAGE (V)
CURRENT (µA)
50
25
75
0
TEMPERATURE (°C)
Shutdown Supply Current
70
60
50
40
30
20
10
2500
V
IN
= 10V
TRANSCONDUCTANCE (µMho)
INPUT SUPPLY CURRENT (µA)
GAIN (µMho)
0
0
0.1
0.2
0.3
SHUTDOWN VOLTAGE (V)
0.4
1959 G07
4
U W
100
125
1959 G04
Switch Peak Current Limit
6.5
6.0
TYPICAL
1.215
Feedback Pin Voltage
1.210
1.205
– 50
–25
0
25
50
75
100
125
TEMPERATURE (°C)
1959 G03
Lockout and Shutdown
Thresholds
25
LOCKOUT
Shutdown Supply Current
V
SHDN
= 0V
20
2.32
15
0.8
START-UP
0.4
SHUTDOWN
0
–50 –25
10
5
0
50
100
25
75
0
JUNCTION TEMPERATURE (°C)
125
0
5
10
INPUT VOLTAGE (V)
15
1959 G06
1959 G05
Error Amplifier Transconductance
3000
Error Amplifier Transconductance
200
PHASE
2500
GAIN
100
V
C
C
OUT
12pF
2000
150
PHASE (DEG)
1500
2000
1000
1500
V
FB
2
×
10
–3
(
)
R
OUT
200k
50
500
1000
ERROR AMPLIFIER EQUIVALENT CIRCUIT
0
R
LOAD
= 50Ω
0
50
0
75 100
25
–50 –25
JUNCTION TEMPERATURE (°C)
125
500
100
1k
10k
100k
FREQUENCY (Hz)
1M
–50
10M
1959 G09
1959 G08
LT1959
TYPICAL PERFORMANCE CHARACTERISTICS
Frequency Foldback
SWITCHING FREQUENCY (kHz) OR CURRENT (µA)
500
SWITCHING
FREQUENCY
550
540
530
400
LOAD CURRENT (A)
FREQUENCY (kHz)
300
200
100
FEEDBACK
PIN CURRENT
0
0
0.4
0.6
0.8
1.0
0.2
FEEDBACK PIN VOLTAGE (V)
1.2
Maximum Load Current
at V
OUT
= 3.3V
4.4
L= 10µH
4.2
BOOST PIN CURRENT (mA)
L= 5µH
LOAD CURRENT (A)
OUTPUT CURRENT (A)
4.0
3.8
3.6
3.4
L= 1.8µH
3.2
3.0
4
6
8
10
12
14
1959 G14
L= 3µH
INPUT VOLTAGE (V)
V
C
Pin Shutdown Threshold
1.4
SHUTDOWN
THRESHOLD VOLTAGE (V)
1.2
SWITCH VOLTAGE (mV)
1.0
0.8
0.6
0.4
–50
–25
100
JUNCTION TEMPERATURE (°C)
*See “More Than Just Voltage Feedback” in the Applications Information section.
U W
1959 • G10
Switching Frequency
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
2.8
–25
0
25
50
75
100
125
2.6
Maximum Load Current
at V
OUT
= 5V
L= 10µH
L= 5µH
520
510
500
490
480
470
460
450
– 50
L= 3µH
L= 1.8µH
5
7
TEMPERATURE (°C)
1959 G11
11
9
INPUT VOLTAGE (V)
13
15
1959 G13
BOOST Pin Current
100
90
80
70
60
50
40
30
20
10
0
0
1
3
2
4
SWITCH CURRENT (A)
5
1959 G15
Current Limit Foldback
7
6
5
4
3
2
MOS LOAD
1
0
0
20
40
60
80
100
1959 G16
DUTY CYCLE = 100%
FOLDBACK
CHARACTERISTICS
POSSIBLE UNDESIRED
CURRENT
STABLE POINT FOR
SOURCE
CURRENT SOURCE
LOAD
LOAD*
RESISTOR
LOAD
OUTPUT VOLTAGE (%)
Switch Voltage Drop
500
450
400
350
300
250
200
150
100
50
25°C
125°C
– 40°C
0
25
50
75
125
0
0
1
2
4
3
SWITCH CURRENT (A)
5
1959 G18
1959 G17
5
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