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

LM22676

LM22676-Q1

www.ti.com

SNVS587K – SEPTEMBER 2008 – REVISED APRIL 2013

SIMPLIFIED BLOCK DIAGRAM

Figure 12. Simplified Block Diagram

Detailed Operating Description

The LM22676 incorporates a voltage mode constant frequency PWM architecture. In addition, input voltage feed-

forward 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 5V 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; 1.285V (typ.).

The functional block diagram of the LM22676 is shown in Figure 12 .

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 1.6V (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 12V. The EN input has an internal pull-up of about 6 µA. Therefore this pin can be

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

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

the 6V absolute maximum specification for this pin.

Copyright © 2008–2013, Texas Instruments Incorporated

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Product Folder Links: LM22676 LM22676-Q1