Datasheet

FB
SW
RT
V
IN
BST
SD
5V
S
R
Q
Q
AGND
IS
CLK
+
SS
PRE
3
2
10
6
5
4 7
8
11
9
12
1
SD
Ir
LM25574
SHUTDOWN
STANDBY
7V
REGULATOR
SYNC
SYNC
OSCILLATOR
RAMP OUT
PGND
CLK
CLK
COMP
ERROR
AMP
R3
21k
C3
470p
C9
22
R6
1.65k
R5
5.11k
L1
100 PH
C7
0.022
C8
0.47
D1
CMSH2-60M
13
15
14
16
THERMAL
SHUTDOWN
UVLO
UVLO
CLK
DIS
VCC
LEVEL
SHIFT
DRIVER
1.225V
1.225V
0.7V
0.7V
R4
24.9k
C5
0.022
C6
open
R2
OPEN
C2
OPEN
C4
0.01
C1
1.0
R1
OPEN
7V ± 42V
VIN
V
IN
1.4V
PWM
C_LIMIT
10 PA
5 PA
V
IN
TRACK
SAMPLE
and
HOLD
2V/A
RAMP GENERATOR
Ir = (10 PA x (V
IN
± V
OUT
))
+ 50 PA
LM25574, LM25574-Q1
SNVS483G –JANUARY 2007–REVISED APRIL 2013 www.ti.com
TYPICAL APPLICATION CIRCUIT AND BLOCK DIAGRAM
Detailed Operating Description
The LM25574 switching regulator features all of the functions necessary to implement an efficient high voltage
buck regulator using a minimum of external components. This easy to use regulator integrates a 42V N-Channel
buck switch with an output current capability of 0.5 Amps. The regulator control method is based on current
mode control utilizing an emulated current ramp. Peak current mode control provides inherent line voltage feed-
forward, cycle-by-cycle current limiting, and ease of loop compensation. The use of an emulated control ramp
reduces noise sensitivity of the pulse-width modulation circuit, allowing reliable processing of very small duty
cycles necessary in high input voltage applications. The operating frequency is user programmable from 50kHz
to 1MHz. An oscillator synchronization pin allows multiple LM25574 regulators to self synchronize or be
synchronized to an external clock. The output voltage can be set as low as 1.225V. Fault protection features
include, current limiting, thermal shutdown and remote shutdown capability. The device is available in the
TSSOP-16 package.
The functional block diagram and typical application of the LM25574 are shown in Typical Application Circuit and
Block Diagram. The LM25574 can be applied in numerous applications to efficiently step-down a high,
unregulated input voltage. The device is well suited for telecom, industrial and automotive power bus voltage
ranges.
High Voltage Start-Up Regulator
The LM25574 contains a dual-mode internal high voltage startup regulator that provides the Vcc bias supply for
the PWM controller and boot-strap MOSFET gate driver. The input pin (VIN) can be connected directly to the
input voltage, as high as 42 Volts. For input voltages below 9V, a low dropout switch connects Vcc directly to
Vin. In this supply range, Vcc is approximately equal to Vin. For Vin voltage greater than 9V, the low dropout
switch is disabled and the Vcc regulator is enabled to maintain Vcc at approximately 7V. The wide operating
range of 6V to 42V is achieved through the use of this dual mode regulator.
The output of the Vcc regulator is current limited to 25mA. Upon power up, the regulator sources current into the
capacitor connected to the VCC pin. When the voltage at the VCC pin exceeds the Vcc UVLO threshold of 5.35V
and the SD pin is greater than 1.225V, the output switch is enabled and a soft-start sequence begins. The output
switch remains enabled until Vcc falls below 5.0V or the SD pin falls below 1.125V.
8 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LM25574 LM25574-Q1