LT1977
High Voltage 1.5A, 500kHz
Step-Down Switching Regulator
with 100µA Quiescent Current
FEATURES
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DESCRIPTIO
Wide Input Range: 3.3V to 60V
1.5A Peak Switch Current
Burst Mode
®
Operation: 100µA Quiescent Current**
Low Shutdown Current: I
Q
< 1µA
Power Good Flag with Programmable Threshold
Load Dump Protection to 60V
500kHz Switching Frequency
Saturating Switch Design: 0.2Ω On-Resistance
Peak Switch Current Maintained Over
Full Duty Cycle Range*
1.25V Feedback Reference Voltage
Easily Synchronizable
Soft-Start Capability
Small 16-Pin Thermally Enhanced TSSOP Package
The LT
®
1977 is a 500kHz monolithic buck switching
regulator that accepts input voltages 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 cir-
cuitry. Current mode topology is used for fast transient
response and good loop stability.
Innovative design techniques along with a new high volt-
age process achieve high efficiency over a wide input
range. Efficiency is maintained over a wide output current
range by employing Burst Mode operation at low currents,
utilizing the output to bias the internal circuitry, and by
using a supply boost capacitor to fully saturate the power
switch. Patented circuitry maintains peak switch current
over the full duty cycle range.* Shutdown reduces input
supply current to less than 1µA. External synchronization
can be implemented by driving the SYNC pin with logic-level
inputs. A single capacitor from the C
SS
pin to the output
provides a controlled output voltage ramp (soft-start). The
device also has a power good flag with a programmable
threshold and time-out and thermal shutdown protection.
The LT1977 is available in a 16-pin TSSOP package with
exposed pad leadframe for low thermal resistance. The
LT1976, a 200kHz reduced switch frequency version of the
LT1977, is also available. See the Applications Information
section for selection criteria between the LT1976 and
LT1977.
Efficiency and Power Loss
vs Load Current
100
V
IN
= 12V
EFFICIENCY
5V
75
EFFICIENCY (%)
APPLICATIO S
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High Voltage Power Conversion
14V and 42V Automotive Systems
Industrial Power Systems
Distributed Power Systems
Battery-Powered Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
*U.S. Patent 6,498,466 and 6,531,909 **See Burst Mode Operation section for conditions
TYPICAL APPLICATIO
Supply Current vs Input Voltage
150
125
SUPPLY CURRENT (µA)
V
OUT
= 3.3V
T
A
= 25°C
14V to 3.3V Step-Down Converter with
100µA No Load Quiescent Current
V
IN
V
IN
SHDN
LT1977
V
C
1500pF
26k
1µF
C
T
SYNC
GND
330pF
C
SS
V
BIAS
FB
PGFB
PG
100k
1%
10pF
165k
1%
+
100µF
6.3V
TANT
BOOST
0.1µF
SW
0.1µF
10µH
1N4148
V
OUT
3.3V
1A
100
75
50
25
0
0
10
30
40
20
INPUT VOLTAGE (V)
50
60
1977 F05
2.2µF
100V
CER
10MQ100N
1977 TA01
U
10
3.3V
1
POWER LOSS (W)
U
U
50
TYPICAL
POWER LOSS
25
0.1
0.01
0
0.0001
0.001
0.1
0.01
LOAD CURRENT (A)
1
0.001
10
1977 TA02
1977f
1
LINER [ LINEAR TECHNOLOGY ]
