Final Electrical Specifications
LT1956/LT1956-5
High Voltage, 1.5A,
500kHz Step-Down
Switching Regulators
June 2001
FEATURES
s
s
s
s
s
DESCRIPTIO
s
s
s
s
s
s
s
Wide Input Range: 5.5V to 60V
1.5A Peak Switch Current
Small 16-Pin SSOP Package
Saturating Switch Design: 0.2Ω
Peak Switch Current Maintained Over
Full Duty Cycle Range
Constant 500kHz Switching Frequency
Effective Supply Current: 2.5mA
Shutdown Current: 25µA
1.2V Feedback Reference (LT1956)
5V Fixed Output (LT1956-5)
Easily Synchronizable
Cycle-by-Cycle Current Limiting
The LT
®
1956/LT1956-5 are 500kHz monolithic buck
switching regulators with an input voltage capability up to
60V. A high efficiency 1.5A, 0.2Ω switch is included on the
die along with all the necessary oscillator, control and logic
circuitry. The topology is current mode for fast transient
response and good loop stability.
Special design techniques and a new high voltage process
achieve high efficiency over a wide input range. Efficiency
is maintained over a wide output current range by using the
output to bias the circuitry and by utilizing a supply boost
capacitor to saturate the power switch. Patented circuitry
maintains peak switch current over the full duty cycle range.
A shutdown pin reduces supply current to 25µA and a SYNC
pin allows the device to be externally synchronized from
570kHz to 700kHz with a logic level input.
The LT1956/LT1956-5 are available in a fused-lead 16-pin
SSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
s
s
s
s
High Voltage, Industrial and Automotive
Portable Computers
Battery-Powered Systems
Battery Chargers
Distributed Power Systems
TYPICAL APPLICATIO
5V Buck Converter
D2
D1N914
6
V
IN
12V
(TRANSIENTS
TO 60V)
BOOST
4
V
IN
LT1956 SW
BIAS
FB
V
C
C
C
1nF
50
100
C2
0.1µF
2
10
12
D1
10MQ060N
L1*
5µH
C3
15
SHDN
2.2µF
50V
CERAMIC 14
SYNC
GND
EFFICIENCY (%)
V
OUT
5V
1A
R1
15.4k
R2
4.99k
+
C1
100µF 10V
SOLID
TANTALUM
1, 8, 9, 16 11
*INCREASE L1 TO 10µH FOR LOAD CURRENTS
ABOVE 0.6A AND TO 18µH ABOVE 1A
1956 TA01
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.
U
Efficiency vs Load Current
V
IN
= 12V
L = 18µH
90
V
OUT
= 5V
V
OUT
= 3.3V
80
70
60
0
0.25
0.75
1.00
0.50
LOAD CURRENT (A)
1.25
1956 TA02
U
U
1
LT1956/LT1956-5
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
GND
SW
NC
V
IN
NC
BOOST
NC
GND
1
2
3
4
5
6
7
8
16 GND
15 SHDN
14 SYNC
13 NC
12 FB/SENSE
11 V
C
10 BIAS
9
GND
Input Voltage (V
IN
) ................................................. 60V
BOOST Pin Above SW ............................................ 35V
BOOST Pin Voltage ................................................. 68V
SYNC, SENSE Voltage (LT1956-5) ........................... 7V
SHDN Voltage ........................................................... 6V
BIAS Pin Voltage .................................................... 48V
FB Pin Voltage/Current (LT1956) ................... 3.5V/2mA
Operating Junction Temperature Range
LT1956EGN/LT1956-5EGN (Note 8) .. – 40°C to 125°C
LT1956IGN/LT1956-5IGN (Note 8) ... – 40°C to 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART
NUMBER
LT1956EGN
LT1956IGN
LT1956-5EGN
LT1956-5IGN
GN PART MARKING
1956
1956I
19565
19565I
GN PACKAGE
16-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 95°C/ W
Consult LTC Marketing for parts specified with wider operating temperature
ranges.
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are at T
J
= 25°C.
V
IN
= 15V, V
C
= 1.5V, SHDN = 1V, Boost o/c, SW o/c, unless otherwise noted.
PARAMETER
Reference Voltage
SENSE Voltage (LT1956-5)
SENSE Pin Resistance
FB Input Bias Current
Error Amp Voltage Gain
Error Amp g
m
V
C
to Switch g
m
EA Source Current
EA Sink Current
V
C
Switching Threshold
V
C
High Clamp
Switch Current Limit
Switch On Resistance
Maximum Switch Duty Cycle
Switch Frequency
f
SW
Line Regulation
f
SW
Shifting Threshold
FB = 1V or V
SENSE
= 4.1V
FB = 1.4V or V
SENSE
= 5.7V
Duty Cycle = 0
SHDN = 1V
V
C
Open, Boost = V
IN
+ 5V, FB = 1V or V
SENSE
= 4.1V
I
SW
= 1.5A, Boost = V
IN
+ 5V (Note 7)
q
q
q
q
q
CONDITIONS
5.5V
≤
V
IN
≤
60V
V
OL
+ 0.2
≤
V
C
≤
V
OH
– 0.2
5.5V
≤
V
IN
≤
60V
V
OL
+ 0.2
≤
V
C
≤
V
OH
– 0.2
q
q
MIN
1.204
1.189
4.94
4.90
9.5
TYP
1.219
5
13.8
0.5
400
2000
1.7
MAX
1.234
1.249
5.06
5.10
19
1.5
3000
3200
400
450
UNITS
V
V
V
V
kΩ
µA
V/V
µMho
µMho
A/V
µA
µA
V
V
(Notes 2, 9)
dl (V
C
) =
±10µA
(Note 9)
q
200
1500
1100
125
100
225
225
0.9
2.1
1.5
2
0.2
3
0.3
0.4
FB = 1V or V
SENSE
= 4.1V
q
82
75
460
430
90
90
500
0.05
0.8
540
570
0.15
V
C
Set to Give DC = 50%
q
5.5V
≤
V
IN
≤
60V
Df = 10kHz
q
2
U
A
Ω
Ω
%
%
kHz
kHz
%/V
V
W
U
U
W W
W
LT1956/LT1956-5
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are at T
J
= 25°C.
V
IN
= 15V, V
C
= 1.5V, SHDN = 1V, Boost o/c, SW o/c, unless otherwise noted.
PARAMETER
Minimum Input Voltage
Minimum Boost Voltage
Boost Current (Note 5)
Input Supply Current (I
VIN
)
Output Supply Current (I
BIAS
)
Shutdown Supply Current
Lockout Threshold
Shutdown Thresholds (LT1956 Only)
Minimum SYNC Amplitude
SYNC Frequency Range
SYNC 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 low
clamp 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 remain constant.
Actual minimum input voltage to maintain a regulated output will depend
upon 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 quiescent current drawn by the input
pin when the BIAS pin is held at 5V with switching disabled. Bias supply
current is the current drawn by the BIAS pin when the BIAS pin is held at
5V. Total input referred supply current is calculated by summing input
supply current (I
VIN
) with a fraction of supply current (I
BIAS
):
I
TOTAL
= I
VIN
+ (I
BIAS
)(V
OUT
/V
IN
)
With V
IN
= 15V, V
OUT
= 5V, I
VIN
= 1.4mA, I
BIAS
= 2.9mA, I
TOTAL
= 2.4mA.
CONDITIONS
(Note 3)
(Note 4) I
SW
≤
1.5A
Boost = V
IN
+ 5V, I
SW
= 0.5A
Boost = V
IN
+ 5V, I
SW
= 1.5A
(Note 6) V
BIAS
= 5V
(Note 6) V
BIAS
= 5V
SHDN = 0V, V
IN
≤
60V, SW = 0V, V
C
Open
q
q
q
q
q
MIN
TYP
4.6
2
12
42
1.4
2.9
25
MAX
5.5
3
25
70
2.2
4.2
75
200
2.46
0.6
0.6
2.2
700
UNITS
V
V
mA
mA
mA
mA
µA
µA
V
V
V
V
kHz
kΩ
V
C
Open
V
C
Open, Shutting Down
V
C
Open, Starting Up
q
q
q
q
2.3
0.15
0.25
580
2.38
0.37
0.45
1.5
20
Note 7:
Switch on resistance is calculated by dividing V
IN
to SW voltage by
the forced current (1.5A). See Typical Performance Characteristics for the
graph of switch voltage at other currents.
Note 8:
The LT1956EGN/LT1956-5EGN are guaranteed to meet
performance specifications from 0°C to 125°C junction temperature.
Specifications over the –40°C to 125°C operating junction temperature
range are assured by design, characterization and correlation with
statistical process controls. The LT1956IGN/LT1956-5IGN are guaranteed
and tested over the full – 40°C to 125°C operating junction temperature
range.
Note 9:
Transconductance and voltage gain refer to the internal amplifier
exclusive of the voltage divider. To calculate gain and transconductance,
refer to the SENSE pin on fixed voltage parts. Divide values shown by the
ratio V
OUT
/1.219.
3
LT1956/LT1956-5
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Peak Current Limit
2.5
1.234
1.229
SWITCH PEAK CURRENT (A)
TYPICAL
2.0
FEEDBACK VOLTAGE (V)
VOLTAGE
1.219
CURRENT
1.214
0.5
1.209
1.0
CURRENT (µA)
1.5
GUARANTEED MINIMUM
1.0
0
20
40
60
DUTY CYCLE (%)
80
100
1956 G01
Lockout and Shutdown
Thresholds
2.4
INPUT SUPPLY CURRENT (µA)
INPUT SUPPLY CURRENT (µA)
2.0
LOCKOUT
SHDN PIN VOLTAGE (V)
1.6
1.2
0.8
START-UP
0.4
SHUTDOWN
0
–50
–25
0
25
50
75
JUNCTION TEMPERATURE (°C)
1956
G04
Error Amplifier Transconductance
2500
TRANSCONDUCTANCE (µmho)
2000
GAIN (µMho)
2500
GAIN
150
PHASE (DEG)
SWITICHING FREQUENCY (kHz)
OR FB CURRENT (µA)
1500
1000
500
0
–50
–25
0
25
50
75
JUNCTION TEMPERATURE
1956
G07
4
U W
100
FB Pin Voltage and Current
2.0
250
200
1.5
1.224
150
100
12
6
0
125
SHDN Pin Bias Current
CURRENT REQUIRED TO FORCE SHUTDOWN
(FLOWS OUT OF PIN). AFTER SHUTDOWN,
CURRENT DROPS TO A FEW
µA
CURRENT (µA)
AT 2.38V STANDBY THRESHOLD
(CURRENT FLOWS OUT OF PIN)
1.204
50
100
25
75
–50 –25
0
JUNCTION TEMPERATURE (°C)
0
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
1956 G02
1956 G03
Shutdown Supply Current
40
35
30
25
20
15
10
5
0
125
Shutdown Supply Current
300
V
SHDN
= 0V
250
V
IN
= 60V
200
V
IN
= 15V
150
100
50
0
0
10
20
30
40
INPUT VOLTAGE (V)
50
60
1956
G05
0
0.1
0.2
0.3
0.4
SHUTDOWN VOLTAGE (V)
0.5
1956
G06
Error Amplifier Transconductance
3000
PHASE
200
Frequency Foldback
625
SWITCHING
FREQUENCY
500
2000
100
375
1500
V
FB
2 • 10
–3
(
)
R
OUT
200k
V
C
C
OUT
12pF
50
250
1000
ERROR AMPLIFIER EQUIVALENT CIRCUIT
0
125
FB PIN
CURRENT
0
0
0.2
0.4
0.6
V
FB
(V)
0.8
1.0
1.2
1956
G09
R
LOAD
= 50Ω
100
125
500
100
1k
10k
100k
FREQUENCY (Hz)
1M
–50
10M
1956
G08
LT1956/LT1956-5
TYPICAL PERFOR A CE CHARACTERISTICS
Switching Frequency
575
550
7.5
BOOST PIN CURRENT (mA)
INPUT VOLTAGE (V)
FREQUENCY (kHz)
525
500
475
450
425
–50
–25
0
25
50
75
JUNCTION TEMPERATURE (°C)
1956
G10
V
C
Pin Shutdown Threshold
2.1
1.9
THRESHOLD VOLTAGE (V)
1.7
1.5
1.3
1.1
0.9
0.7
50
100
–50 –25
25
75
0
JUNCTION TEMPERATURE (°C)
SWITCH VOLTAGE (mV)
U W
100
Minimum Input Voltage with 5V
Output
V
OUT
= 5V
L = 18µH
45
40
35
30
25
20
15
10
5
125
5.0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
LOAD CURRENT (A)
1
0
BOOST Pin Current
7.0
MINIMUM INPUT
VOLTAGE TO START
6.5
6.0
MINIMUM INPUT
VOLTAGE TO RUN
5.5
0
0.5
1
SWITCH CURRENT (A)
1.5
1956 G12
1956
G11
Switch Voltage Drop
450
400
350
300
250
200
150
100
50
125
T
J
= 125°C
T
J
= 25°C
T
J
= –40°C
0
0
0.5
1
SWITCH CURRENT (A)
1.5
1766 G14
1956
G13
5
FPGA/CPLD
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