MCP1650 datasheet(13/28 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

DS21876A-page 13

MCP1650/51/52/53

4.0

DETAILED DESCRIPTION

4.1

Device Overview

The MCP1650/51/52/53 is a gated oscillator boost

controller. By adding an external N-channel MOSFET,

schottky diode and boost inductor, high-output power

applications can be achieved. The 750 kHz hysteretic

gated oscillator architecture enables the use of small,

low-cost external components. By using a hysteretic

approach,

compensation

components

are

necessary for the stability of the regulator output.

Output

voltage

regulation

accomplished

comparing the output voltage (sensed through an

external resistor divider) to a reference internal to the

MCP1650/51/52/53. When the sensed output voltage

is below the reference, the EXT pin pulses the external

N-channel MOSFET on and off at the 750 kHz gated

oscillator frequency. Energy is stored in the boost

inductor when the external N-channel MOSFET is on

and is delivered to the load through the external

Schottky diode when the MOSFET is turned off.

Several pulses may be required to deliver enough

energy to pump the output voltage above the upper

hysteretic limit. Once above the hysteretic limit, the

internal oscillator is no longer gated to the EXT pin and

no energy is transferred from input to output.

The peak current in the MOSFET is sensed to limit its

maximum value. As with all boost topology converters,

even though the MOSFET is turned off, there is still a

DC path through the boost inductor and diode to the

load. Additional protection circuity, such as fuses, are

recommended for short circuit protection.

4.2

Input Voltage

The range of input voltage for the MCP1650/51/52/53

family of devices is specified from 2.7V to 5.5V. For the

S-option devices, the undervoltage lockout (UVLO)

feature will turn the boost controller off once the input

voltage falls below 2.55V, typical. For the R-option

devices, the UVLO is set to 2.0V. The R-option devices

are recommended for use when “bootstrapping” the

output voltage back to the input. The input of the

MCP1650/51/52/53 device is supplied by the output

voltage during boost operation. This can be used to

derive output voltages from input voltages that start up

at approximately 2V (2-cell alkaline batteries).

4.3

Fixed Duty Cycle

The MCP1650/51/52/53 family utilizes a unique two-

step maximum duty cycle architecture to minimize input

peak current and improve output ripple voltage for wide

input voltage operating ranges. When the input voltage

is below 3.8V, the duty cycle is typically 80%. For input

voltages above 3.8V, the duty cycle is typically 56%. By

decreasing the duty cycle at higher input voltages, the

input peak current is reduced. For low input voltages, a

longer duty cycle stores more energy during the on-

time of the boost MOSFET. For applications that span

the 3.8V input range, the inductor value should be

selected to meet not only the minimum input voltage at

80% duty cycle, but 3.8V at 56% duty cycle as well.

Refer to Section 5.0 “Application Circuits/Issues”

for more information about selecting inductor values.

4.4

Shutdown Input Operation

The SHDN pin is used to turn the MCP1650/51/52/53

on and off. When the SHDN pin is tied low, the

MCP1650/51/52/53 is off. When tied high, the

MCP1650/51/52/53 will be enabled and begin boost

operation as long as the input voltage is not below the

UVLO threshold.

4.5

Soft-Start Operation

When power is first applied to the MCP1650/51/52/53,

the internal reference initialization is controlled to slow

down the start-up of the boost output voltage.This is

done to reduce high inrush current required from the

source. High inrush currents can cause the source

voltage to drop suddenly and trip the UVLO threshold,

shutting down the converter prior to it reaching steady-

state operation.

4.6

Gated Oscillator Architecture

A 750 kHz internal oscillator is used as the base

frequency of the MCP1650/51/52/53. The oscillator

duty cycle is typically 80% when the input voltage is

below a nominal value of 3.8V, and 56% when the

input voltage is above a nominal value of 3.8V. Two

duty cycles are provided to reduce the peak inductor

current in applications where the input voltage varies

over a wide range. High-peak inductor current results

in undesirable high-output ripple voltages. For

applications that have input voltage that cross this

3.8V boundary, both duty cycle conditions need to be

examined to determine which one has the least

amount of energy storage. Refer to Section 5.0

“Application Circuits/Issues” for more information

about design considerations.

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