MCP1253 datasheet(10/18 Pages) MICROCHIP

10 / 18 page

MCP1252/3

DS21752A-page 10

 2002 Microchip Technology Inc.

4.2

Power Efficiency

The power efficiency,

η, is determined by the mode of

operation. In boost mode, the efficiency is approxi-

mately half of a linear regulator. In buck mode, the effi-

ciency is approximately equal to that of a linear

regulator. The following formulas can be used to

approximate the power efficiency with any significant

amount of output current. At light loads, the quiescent

current of the device must be taken into consideration.

EQUATION

4.3

Shutdown Mode

Driving SHDN low places the MCP1252 or MCP1253 in

a low power shutdown mode. This disables the charge

pump switches, oscillator and control logic, reducing

the quiescent current to 0.1 µA (typical). The PGOOD

output is in a high-impedance state during shutdown.

4.4

PGOOD Output

The PGOOD output is an open-drain output that sinks

current when the regulator output voltage falls below

0.93VOUT (typical). The output voltage can either be

fixed when the selectable output device is chosen

(MCP1252-33X50, MCP1253-33X50) or adjustable

from an external resistive divider when the adjustable

device is chosen (MCP1252-ADJ, MCP1253-ADJ). If

the regulator output voltage falls below 0.93VOUT (typ-

ical) for less than 200 µsec and then recovers, glitch-

immunity circuits prevent the PGOOD signal from tran-

sitioning low. A 10 k

Ω to 1 MΩ pull-up resistor from

PGOOD to VOUT may be used to provide a logic output.

Connect PGOOD to GND or leave unconnected if not

used.

4.5

Soft-Start and Short-Circuit

Protection

The MCP1252 and MCP1253 features foldback short-

circuit protection. This circuitry provides an internal

soft-start function by limiting in-rush current during

startup and also limits the output current to 200 mA

(typical) if the output is shorted to GND. The internal

soft-start circuitry requires approximately 300 µsec,

typical with a 5V output, from either initial power-up or

release from shutdown for the output voltage to be in

regulation.

4.6

Thermal Shutdown

The MCP1252 and MCP1253 feature thermal shut-

down with temperature hysteresis. When the die tem-

perature exceeds 160°C, typically, the device shuts

down. When the die cools by 15°C, typically, the device

automatically turns back on. If high die temperature is

caused by output overload and the load is not removed,

the device will turn on and off, resulting in a pulse out-

put.

5.0

APPLICATIONS

The MCP1252 and MCP1253 are inductorless, positive

regulated, charge pump DC/DC converters. A typical

circuit configuration for the fixed output version is

depicted in Figure 5-1. The adjustable version is

depicted in Figure 5-2.

FIGURE 5-1:

Typical Circuit Configuration

for Fixed Output Device.

FIGURE 5-2:

Typical Circuit Configuration

for Adjustable Output Device.

ηBOOST

POUT

-------------

VOUT IOUT

VIN 2

IOUT

------------------------------------

VOUT

VIN 2

------------------

ηBUCK

POUT

-------------

VOUT IOUT

VIN IOUT

--------------------------------

VOUT

VIN

-------------

MCP1252-33X50

C+

CFLY

C-

VIN

SHDN

OFF

Shutdown

Control

2.7V to 5.5V

CIN

GND

SELECT

PGOOD

VOUT

COUT

RPU

PGOOD Flag

To PICmicro®

+5.0V ±2.5%

SELECTABLE OUTPUT VOLTAGE

CFLY = 1µF

CIN = 10 µF

COUT = 10 µF

Microcontroller

RPU = 100 kΩ

MCP1252-ADJ

C+

CFLY

C-

VIN

SHDN

OFF

Shutdown

Control

2.7V to 5.5V

CIN

GND

PGOOD

VOUT

COUT

RPU

PGOOD Flag

To PICmicro®

+4.0V

ADJUSTABLE OUTPUT VOLTAGE

VOUT = 1.21V (1 + R1/R2)

Microcontroller

CFLY = 1µF

CIN = 10 µF

COUT = 10 µF

RPU = 100 kΩ

R1 = 23.2 kΩ

R2 = 10 kΩ

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