LOGIC

OSC

+

-

TYPE III

COMP

+

-

1.285V

&

Soft-start

INT REG, EN,UVLO

GND

SW

BOOT

VIN

EN

FB

VIN

VOUT

Error Amp.

PWM Cmp.

ILimit

LM22674, LM22674-Q1

SNVS590M – SEPTEMBER 2008 – REVISED NOVEMBER 2014

www.ti.com

7 Detailed Description

7.1 Overview

The LM22674 incorporates a voltage mode constant frequency PWM architecture. In addition, input voltage

feedforward is used to stabilize the loop gain against variations in input voltage. This allows the loop

compensation to be optimized for transient performance. The power MOSFET, in conjunction with the diode,

produce a rectangular waveform at the switch pin, that swings from about zero volts to VIN. The inductor and

output capacitor average this waveform to become the regulator output voltage. By adjusting the duty cycle of

this waveform, the output voltage can be controlled. The error amplifier compares the output voltage with the

internal reference and adjusts the duty cycle to regulate the output at the desired value.

The internal loop compensation of the -ADJ option is optimized for outputs of 5V and below. If an output voltage

of 5 V or greater is required, the -5.0 option can be used with an external voltage divider. The minimum output

voltage is equal to the reference voltage, that is, 1.285 V (typ).

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Precision Enable and UVLO

The precision enable input (EN) is used to control the regulator. The precision feature allows simple sequencing

of multiple power supplies with a resistor divider from another supply. Connecting this pin to ground or to a

voltage less than 1.6 V (typ) will turn off the regulator. The current drain from the input supply, in this state, is

25 µA (typ) at an input voltage of 12 V. The EN input has an internal pullup of about 6 µA. Therefore this pin can

be left floating or pulled to a voltage greater than 2.2 V (typ) to turn the regulator on. The hysteresis on this input

is about 0.6 V (typ) above the 1.6 V (typ) threshold. When driving the enable input, the voltage must never

exceed the 6 V absolute maximum specification for this pin.

8

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