MCP1662T-ESLASHOT datasheet(9/28 Pages) MICROCHIP

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DS20005316E-page 9

MCP1662

4.0

DETAILED DESCRIPTION

4.1

Device Overview

The MCP1662 device is a fixed-frequency, synchro-

nous step-up converter, with a low-voltage reference of

300 mV, optimized to keep the output current constant

by regulating the voltage across the feedback resistor

(RSET). The MCP1662 integrates a peak current mode

architecture. It delivers high-efficiency conversion for

an LED lighting application when it is powered by two-

or three-cell alkaline, lithium, NiMH, NiCd, or single-cell

Lithium-Ion batteries. The maximum input voltage is

5.5V. A high level of integration lowers total system

cost, eases implementation and reduces board area.

The conventional boost converter with a high-voltage

reference has a high-voltage drop across the LED

series current limit resistor. The power dissipated in this

resistor, which is usually in series with the LED string,

reduces the total efficiency conversion of an LED driver

solution. Therefore, the voltage drop on the sense

resistor (RSET) that is used to regulate the LED current

must be low. In the case of MCP1662, the VFB value is

300 mV.

The

device

features

controlled

start-up

voltage

(UVLOSTART = 2.3V) and open load protection, in case

the LED fails or a short circuit of the VFB pin to GND

occurs. If the VFB voltage drops to 50 mV typical, the

device stops switching and the output voltage will be

equal to the input voltage (minus a diode drop voltage).

This feature prevents damage to the device and LEDs

when there is an accidental drop in voltage.

The 800 m

, 36V integrated switch is protected by the

1.3A cycle-by-cycle inductor peak current limit opera-

tion. When the Enable pin is pulled to ground

(EN = GND), the device stops switching, enters into

Shutdown mode and consumes less than 50 nA of

input current (Figure 2-8).

4.2

Functional Description

The MCP1662 is a compact, high-efficiency, fixed

500 kHz frequency, step-up DC-DC converter. It oper-

ates as a constant current generator for applications

gizer® batteries, or three-cell NiCd or NiMH batteries,

or one-cell Lithium-Ion or Li-Polymer batteries.

Figure 4-1 depicts the functional block diagram of the

MCP1662. It incorporates a Current mode control

scheme, in which the PWM ramp signal is derived from

the NMOS power switch current (VSENSE). This ramp

signal adds a slope ramp compensation signal (VRAMP)

and is compared to the output of the error amplifier

(VERROR) to control the “on” time of the power switch.

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