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LT1777IS#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
22
LT1777
TYPICAL APPLICATIONS
U
Figure 14. Output Node Ripple
2µs/DIV
(b) After Output Filter
2µs/DIV
1777 F13a
(a) Before Output Filter
V
OUT
NODE
AC COUPLED
2mV/DIV
1777 F14b
User Programmable Undervoltage Lockout
Figure 15 uses a resistor divider between V
IN
and ground
to drive the SHDN node. This is a simple, cost-effective
way to add a user-programmable undervoltage lockout
(UVLO) function. Resistor R5 is chosen to have approxi-
mately 200µA through it at the nominal SHDN pin lockout
threshold of roughly 1.25V. The somewhat arbitrary value
of 200µA was chosen to be significantly above the SHDN
pin input current to minimize its error contribution, but
significantly below the typical 2.5mA the LT1777 draws in
lockout mode. Resistor R4 is then chosen to yield this
same 200µA, less 2.5µA, with the desired V
IN
UVLO volt-
age minus 1.25V across it. (The 2.5mA factor is an allow-
ance to minimize error due to SHDN pin input current.)
Behavior is as follows: Normal operation is observed at the
nominal input voltage of 24V. As the input voltage is
decreased to roughly 18V, switching action will stop, V
OUT
will drop to zero, and the LT1777 will draw its V
IN
and V
CC
quiescent currents from the V
IN
supply. At a lower input
voltage, typically 10V or so at 25°C, the voltage on the
SHDN pin will drop to the shutdown threshold, and the part
will draw its shutdown current only from the V
IN
rail. The
resistive divider of R4 and R5 will continue to draw power
from V
IN
. (The user should be aware that while the SHDN
pin
lockout
threshold is relatively accurate including
temperature effects, the SHDN pin shutdown threshold is
more coarse, and exhibits considerably more temperature
drift. Nevertheless the
shutdown
threshold will always be
well below the lockout threshold.)
Figure 15. User Programmable UVLO
R4
84.5k
1%
R5
6.19k
1%
C5
100pF
SHDN
LT1777
1777 F15
V
IN
V
OUT
NODE
AC COUPLED
2mV/DIV
23
LT1777
PACKAGE DESCRIPTION
U
Dimensions in inches (millimeters) unless otherwise noted.
S Package
16-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.016 – 0.050
(0.406 – 1.270)
0.010 – 0.020
(0.254 – 0.508)
×
45
°
0
°
– 8
°
TYP
0.008 – 0.010
(0.203 – 0.254)
1
2
3
4
5
6
7
8
0.150 – 0.157**
(3.810 – 3.988)
16
15
14
13
0.386 – 0.394*
(9.804 – 10.008)
0.228 – 0.244
(5.791 – 6.197)
12
11
10
9
S16 1098
0.053 – 0.069
(1.346 – 1.752)
0.014 – 0.019
(0.355 – 0.483)
TYP
0.004 – 0.010
(0.101 – 0.254)
0.050
(1.270)
BSC
DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
*
**
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.
24
LT1777
LINEAR TECHNOLOGY CORPORATION 1999
1777f LT/TP 0899 4K • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear-tech.com
circuit is capable of delivering up to 300mA at 5V, from
input voltages as high as 28V. The only disadvantage is
that due to the increased resistance in the inductor, the
circuit is no longer capable of withstanding indefinite short
circuits to ground. The LT1777 will still current limit at its
nominal I
LIM
value, but this will overheat the inductor.
Momentary short circuits of a few seconds or less can still
be tolerated.
TYPICAL APPLICATION
U
Minimum PC Board Size Application
The previously described basic applications employ power
path parts which are capable of delivering the full rated
input supply voltage and output current capabilities of the
LT1777. A substantial improvement in printed circuit
board area requirements can be achieved with the circuit
shown below. This uses a physically smaller and less
costly power inductor and a tantalum input capacitor. This
PART NUMBER DESCRIPTION COMMENTS
LT1076 100kHz, 2A Step-Down Switching Regulator Integrated 2A Switch, V
IN
Up to 46V
LT1533 Ultralow Noise 1A Switching Regulator Push-Pull Design for Low Noise Isolated Supplies
LT1534 Ultralow Noise 2A Switching Regulator Ultralow Noise Regulator for Boost Topologies
LT1576 200kHz, 1.5A Step-Down Switching Regulator Output Up to 1.25A, Integrated Switch, SO-8 Package
LTC1622 Low V
IN
Step-Down DC/DC Controller Fixed Frequency 550kHz Operation, MSOP Package
LTC1624 High Efficiency SO-8 DC/DC Controller 200kHz Operation, V
IN
from 3.5V to 36V, SO-8 Package
LT1676/LT1776 Wide Input Range, High Efficiency, Step-Down Voltage Regulator 7.4V to 60V Input, 100/200kHz Operation, 700mA Internal Switch
RELATED PARTS
+
V
IN
V
CC
V
SW
LT1777
V
D
SHDN
SYNC
4
10
7
6
D1
L1
0µH TO 2.2µH
(SEE BELOW)
L2
200µH
C2
100µF
10V
R1
36.5k
1%
1777 TA03
V
OUT
5V
R2
12.1k
1%
5
3
V
IN
10V TO 28V
12
14
13
V
C
FB
SGND
+
C1
22µF
35V
R3
12k
C3
2200pF
C4
100pF
C1: AVX E CASE TPSE226M035R0300
C2: AVX D CASE TPSD107M010R0080
C3, C4, C5: NPO OR X7R
C6, C7: Z5U
D1: MOTOROLA 100V, 1A SMD SCHOTTKY
MBRS1100
L1: SENSE INDUCTOR CAN VARY FROM 0µH TO 2.2µH
AS PER APPLICATION. SEE PREVIOUS SCHEMATICS
FOR EXAMPLES
L2: COILCRAFT CTX200-1 OR SIMILAR
C5
100pF
C6
0.1µF
C7
0.1µF
Minimum PC Board Area Application
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LT1777IS#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators L N Buck Sw Reg
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