MCR03EZPFX1002 datasheet(13/36 Pages) IRF

13 / 36 page

IR3837MPbF

Rev 1.31

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 IR3837 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 IR3837 does

not turn on until the bus voltage reaches the

desired level (Fig. 3). Only after the bus voltage

reaches or exceeds this level will the voltage at

Enable pin exceed its threshold, thus enabling

the IR3837. Therefore, in addition to being a logic

input pin to enable the IR3837, the Enable

feature, with its precise threshold, also allows the

user to implement an Under-Voltage Lockout for

the bus voltage (PVin).

This is desirable

particularly for high output voltage applications,

where we might want the IR3837 to be disabled

at least until PVin exceeds the desired output

voltage level.

Figure 4a. shows the recommended start-up

sequence for the normal (non-tracking, non-

sequencing) operation of IR3837, when Enable is

used as a logic input. In this operating mode Vref

left

floating.

Figure

4b.

shows

the

recommended startup sequence for sequenced

operation of IR3837 with Enable used as logic

input. For this mode of operation, Vref is left

floating. Figure 4c shows the recommended

startup sequence for tracking operation of

IR3837 with Enable used as logic input. For this

mode of operation, Vref is connected to a voltage

greater than 1.5V.

Circuit Description

THEORY OF OPERATION

Introduction

The IR3837 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.

IR3837 provides precisely regulated output

voltage programmed via two external resistors

from 0.6V to 0.9*Vin.

The IR3837 operates with an internal bias supply

voltage of 5.2V (LDO) which is connected to the

Vcc/LDO_out pin. This allows operation with

single supply. The IC can also be operated with

an external supply from 4.5V to 6.5V, allowing an

extended operating input voltage (PVin) range

from 1.5V to 16V. For using the internal supply,

the Vin pin should be connected to PVin pin. If an

external supply is used, it should be connected to

Vcc/LDO_out pin and the Vin pin should be left

floating.

The device utilizes the on-resistance of the low

side MOSFET (sync FET) as current

sense

element. This method enhances the converter’s

efficiency and reduces cost by eliminating the

need for external current sense resistor

IR3837 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

voltage of Vcc/Ldo pin 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/LDO 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).

Fig. 3. Normal Start up, device turns on

when the bus voltage reaches 10.2V

Pvin (12V)

Vcc (5.2V)

Enable

10. 2 V

Enable Threshold= 1.2V

MCR03EZPFX1002 数据表(PDF) 13 Page - International Rectifier

MCR03EZPFX1002 数据表(HTML) 13 Page - International Rectifier

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

数据表下载