FB

SW

RT

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

LM5574

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

CMSH2-100M

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 ± 75V

VIN

1.4V

PWM

C_LIMIT

10 PA

5 PA

TRACK

SAMPLE

and

HOLD

2V/A

RAMP GENERATOR

+ 50 PA

LM5574, LM5574-Q1

SNVS478F – JANUARY 2007 – REVISED APRIL 2013

www.ti.com

TYPICAL APPLICATION CIRCUIT AND BLOCK DIAGRAM

Figure 9. Functional Block Diagram

Detailed Operating Description

The LM5574 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 75V 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 500kHz. An oscillator synchronization pin allows multiple LM5574 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 LM5574 are shown in Figure 9. The LM5574 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 LM5574 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 75 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 75V 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

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