MCR03EZPFX4021 datasheet(12/35 Pages) IRF

12 / 35 page

Rev 4.0

PD-97516

IR3853MPbF

Circuit Description

THEORY OF OPERATION

Introduction

The IR3853 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 1.5MHz and provides the capability of

optimizing the design in terms of size and

performance.

IR3853 provides precisely regulated output

voltage programmed via two external resistors

from 0.7V to 0.9*Vin.

The IR3853 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

IR3853 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 IR3853 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 IR3853 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 IR3853.

Therefore, in addition to being a logic input pin to

enable the IR3853, 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 IR3853 to be disabled at least until V

exceeds the desired output voltage level.

Figure 3b. shows the recommended start-up

sequence for the non-sequenced operation of

IR3853, when Enable is used as a logic input.

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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