MCP1643-ISLASHMS datasheet(9/28 Pages) MICROCHIP

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DS20005208B-page 9

MCP1643

4.0

DETAILED DESCRIPTION

4.1

Device Overview

The MCP1643 is capable of starting up with a low volt-

age, while achieving high efficiency to drive one or

more LEDs with constant current.

The MCP1643 is a fixed frequency, synchronous

step-up converter, with a low voltage reference of

120 mV, optimized to keep the output current constant

by regulating the voltage across the feedback resistor

(RSET).

The normal boost converter with a high voltage refer-

ence has a high voltage drop across the current sense

resistor. The power dissipated in the sense resistor

reduces the efficiency of a LED driver solution. There-

fore, the voltage drop on the sense resistor used to reg-

ulate the LED current must be low, in this case by a low

VFB value of 120 mV.

The device can operate from one or two-cell alkaline

and NiMH/NiCd batteries. The maximum input voltage

is 5.0V. The device features an Overvoltage Protection

that protects the device if the output voltage (VOUT) is

higher than 5.0V. This usually happens if the LED is

disconnected. While VIN <VOUT, the load current (ILED)

remains in regulation until VIN is close to VLED (see

Typical Applications and Figures 2-1 to 2-3).

A True Output Load Disconnect mode provides input-

to-output isolation while in Shutdown (EN = GND). In

this state, the MCP1643 LED driver drains 1.2 µA cur-

rent from the battery at room temperature.

A high level of integration lowers the total system cost,

eases the implementation and reduces board area.

The device also features internal compensation, low

noise, soft start and thermal shutdown.

4.2

Functional Description

The MCP1643 is a compact, high-efficiency, fixed fre-

quency, step-up DC-DC converter that operates as a

constant current generator for applications powered by

either one or two-cell, alkaline, NiCd, or NiMH batter-

ies.

Figure 4-1 depicts the functional block diagram of the

MCP1643 device.

FIGURE 4-1:

MCP1643 Block Diagram.

Gate Drive

and

Shutdown

Control

Logic

VIN

VOUT

PGND

ISENSE

IZERO

ILIMIT

SOFT-START

Direction

Control

Oscillator

Slope

Compensation

PWM/PFM

Logic

120 mV

Internal

BIAS

VFB

SGND

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