PCMB065T-2R2MS datasheet(12/33 Pages) IRF

12 / 33 page

Rev 16.0

PD-97509

IR3843AMPbF

Circuit Description

THEORY OF OPERATION

Introduction

The IR3843A uses a PWM voltage mode control

scheme with external compensation to provide

good noise immunity and maximum flexibility in

selecting inductor values and capacitor types.

The switching frequency is programmable from

250kHz to 1.2MHz and provides the capability of

optimizing the design in terms of size and

performance.

IR3843A provides precisely regulated output

voltage programmed via two external resistors

from 0.7V to 0.9*Vin.

The IR3843A operates with an external bias

supply from 4.5V to 5.5V, allowing an extended

operating input voltage range from 1.5V to 21V.

The device utilizes the on-resistance of the low

side MOSFET as current sense element, this

method enhances the converter’s efficiency and

reduces cost by eliminating the need for external

current sense resistor

IR3843A includes two low R

ds(on) MOSFETs

using IR’s HEXFET technology. These are

specifically

designed

for

high

efficiency

applications.

Under-Voltage Lockout and POR

The under-voltage lockout circuit monitors the

input supply Vcc and the Enable input. It assures

that the MOSFET driver outputs remain in the off

state whenever either of these two signals drop

below the set thresholds. Normal operation

resumes once Vcc and Enable rise above their

thresholds.

The POR (Power On Ready) signal is generated

when all these signals reach the valid logic level

(see system block diagram). When the POR is

asserted the soft start sequence starts (see soft

start section).

Enable

The Enable features another level of flexibility for

start up. The Enable has precise threshold which

is internally monitored by Under-Voltage Lockout

(UVLO) circuit. Therefore, the IR3843A will turn

on only when the voltage at the Enable pin

exceeds this threshold, typically, 1.2V.

If the input to the Enable pin is derived from the

bus voltage by a suitably programmed resistive

divider, it can be ensured that the IR3843A does

not turn on until the bus voltage reaches the

desired level. Only after the bus voltage reaches

or exceeds this level will the voltage at Enable

pin exceed its threshold, thus enabling the

IR3843A. Therefore, in addition to being a logic

input pin to enable the IR3843A, the Enable

feature, with its precise threshold, also allows the

user to implement an Under-Voltage Lockout for

the bus voltage V

in. This is desirable particularly

for high output voltage applications, where we

might want the IR3843A to be disabled at least

until V

in exceeds the desired output voltage level.

Figure 3b. shows the recommended start-up

sequence for the non-sequenced operation of

IR3843A.

Figure 3c. shows the recommended startup

sequence for sequenced operation of IR3843A

Fig. 3a. Normal Start up, Device turns on

when the Bus voltage reaches 10.2V

Fig. 3b. Recommended startup sequence,

Non-Sequenced operation

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