L7987
61 V 3 A asynchronous step-down switching regulator with
adjustable current limitation
Datasheet
-
production data
Description
The L7987 device is a step-down monolithic
switching regulator able to deliver up to 3 A DC.
The output voltage adjustability ranges from 0.8 V
to V
IN
. The wide input voltage range and the
100% duty cycle capability meet the fail safe
specifications for industrial systems. The
embedded switchover feature on the V
BIAS
pin
maximizes the efficiency at light load. The
adjustable current limitation, designed to select
the inductor RMS current accordingly with the
nominal output current, and the high switching
frequency capability make the size of the
application compact. Pulse-by-pulse current
sensing with digital frequency foldback
implements an effective constant current
protection over the different application
conditions. The peak current foldback decreases
the stress of the power components in heavy
short-circuit condition. The PGOOD open
collector output can also implement output
voltage sequencing during the power-up phase.
Multiple devices can be synchronized sharing the
SYNCH pin to prevent beating noise in low noise
applications like sensors with A/D conversion.
HTSSOP16 (R
TH
= 40 °C/W)
Features
3 A DC output current
4.5 V to 61 V operating input voltage
R
DS,ON
= 250 m typ.
Adjustable f
SW
(250 kHz - 1.5 MHz)
Low IQ-SHD (11 µA typ. from V
IN
)
Low IQ (1 mA typ. - V
IN
24 V - V
OUT
3.3 V)
Output voltage adjustable from 0.8 V to V
IN
Synchronization
Adjustable soft-start time
Adjustable current limitation
Low dropout operation (12 µs max.)
V
BIAS
improves efficiency at light load
PGOOD open collector output
Output voltage sequencing
Digital frequency foldback in short-circuit
Peak current foldback in short-circuit
Auto-recovery thermal shutdown
Applications
Designed for 24 V bus
Fail safe tolerant system
Programmable logic controllers (PLCs)
March 2015
This is information on a product in full production.
DocID025589 Rev 3
1/37
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Contents
L7987
Contents
1
2
Application schematic and block diagram . . . . . . . . . . . . . . . . . . . . . . . 4
Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1
2.2
2.3
2.4
2.5
Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
ESD protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3
4
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1
4.2
4.3
4.4
4.5
4.6
Oscillator and synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Error amplifier and light-load management . . . . . . . . . . . . . . . . . . . . . . . 16
Low VIN operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Overtemperature protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.1
5.2
5.3
5.4
Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.4.1
5.4.2
Type II compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Type III compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.5
5.6
Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
6
7
Demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2/37
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