LT1956/LT1956-5
High Voltage, 1.5A,
500kHz Step-Down
Switching Regulators
U
FEATURES
s
s
s
s
s
s
s
s
s
s
s
s
DESCRIPTIO
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. A current mode architecture provides fast transient response and good loop stability.
Wide Input Range: 5.5V to 60V
1.5A Peak Switch Current
Small 16-Pin SSOP or Thermally Enhanced
TSSOP 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
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
the device can be externally synchronized from 580kHz to
700kHz with a logic level input.
U
APPLICATIO S
s
s
s
s
The LT1956/LT1956-5 are available in fused-lead 16-pin
SSOP and thermally enhanced TSSOP packages.
High Voltage, Industrial and Automotive
Portable Computers
Battery-Powered Systems
Battery Chargers
Distributed Power Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
*U.S. PATENT NO. 6,498,466
U
s
TYPICAL APPLICATIO
5V Buck Converter
Efficiency vs Load Current
MMSD914TI
100
VIN = 12V
L = 18µH
6
BOOST
4
2.2µF†
100V
CERAMIC
SW
VIN
2
10MQ060N
LT1956-5
15
14
SHDN
SYNC
GND
10µH
BIAS
FB
10
VOUT
5V
1A
22µF
6.3V
CERAMIC
90
EFFICIENCY (%)
VIN
12V
(TRANSIENTS
TO 60V)
0.1µF
VOUT = 5V
VOUT = 3.3V
80
70
12
60
VC
1, 8, 9, 16 11
50
220pF
4.7k
0
0.25
0.75
1.00
0.50
LOAD CURRENT (A)
1.25
1956 TA02
4700pF
†
UNITED CHEMI-CON THCS50EZA225ZT
1956 TA01
1956f
1
LT1956/LT1956-5
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TJ = 25°C.
VIN = 15V, VC = 1.5V, SHDN = 1V, Boost o/c, SW o/c, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
Switch Current Limit
VC Open, Boost = VIN + 5V, FB = 1V or VSENSE = 4.1V q
1.5
2
3
A
Switch On Resistance
ISW = 1.5A, Boost = VIN + 5V (Note 7)
0.2
0.3
0.4
Ω
Ω
q
Maximum Switch Duty Cycle
q
82
75
90
90
460
430
500
q
Switch Frequency
FB = 1V or VSENSE = 4.1V
UNITS
540
570
kHz
kHz
0.05
0.15
%/V
VC Set to Give DC = 50%
%
%
fSW Line Regulation
5.5V ≤ VIN ≤ 60V
fSW Shifting Threshold
Df = 10kHz
Minimum Input Voltage
(Note 3)
q
Minimum Boost Voltage
(Note 4) ISW ≤ 1.5A
q
2
3
V
Boost Current (Note 5)
Boost = VIN + 5V, ISW = 0.5A
Boost = VIN + 5V, ISW = 1.5A
q
q
12
42
25
70
mA
mA
Input Supply Current (IVIN)
(Note 6) VBIAS = 5V
1.4
2.2
mA
Output Supply Current (IBIAS)
(Note 6) VBIAS = 5V
2.9
4.2
mA
Shutdown Supply Current
SHDN = 0V, VIN ≤ 60V, SW = 0V, VC Open
25
75
200
µA
µA
q
0.8
4.6
q
V
5.5
V
Lockout Threshold
VC Open
q
2.30
2.42
2.53
V
Shutdown Thresholds
VC Open, Shutting Down
VC Open, Starting Up
q
q
0.15
0.25
0.37
0.45
0.6
0.6
V
V
1.5
2.2
V
700
kHz
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 VC 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 (IVIN) with a fraction of supply current (IBIAS):
ITOTAL = IVIN + (IBIAS)(VOUT/VIN)
with VIN = 15V, VOUT = 5V, IVIN = 1.4mA, IBIAS = 2.9mA, ITOTAL = 2.4mA.
q
580
20
kΩ
Note 7: Switch on resistance is calculated by dividing VIN 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 LT1956EFE/LT1956EFE-5/LT1956EGN/LT1956EGN-5 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 LT1956IFE/LT1956IFE-5/
LT1956IGN/LT1956IGN-5 are guaranteed 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 VOUT/1.219.
Note 10: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
1956f
3
