MCP1643-ISLASHMS datasheet(10/28 Pages) MICROCHIP

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MCP1643

DS20005208B-page 10

 2013-2023 Microchip Technology Inc. and its subsidiaries

4.2.1

LOW-VOLTAGE START-UP

The MCP1643 LED Constant Current Driver is capable

of starting from a low-input voltage. Start-up voltage is

typically 0.65V for a 25 mA LED load.

For applications in which the device turns on and off

fast, the start-up voltage is lower than 0.65V, because

the output capacitor remains partially charged. After

start-up, the device operates down to 0.5V input.

There is no Undervoltage-Lockout feature for the

MCP1643 LED Constant Current Driver. The device

will start up at the lowest possible voltage and run down

to the lowest possible voltage.

When enabled, the internal start-up logic turns the

rectifying P-Channel switch on until the output

capacitor is charged to a value close to the input

voltage. The rectifying switch is current limited during

this time. After charging the output capacitor to the

input voltage, the device starts switching in open loop,

because the LED is turned off and the feedback input

voltage is zero. Once VOUT is equal to the minimum for-

ward voltage (VF) of the LED, the device enters in close

loop and regulates the voltage across the RSET resis-

tor, which is connected between VFB pin and GND.

4.2.2

PWM MODE OPERATION

The MCP1643 LED Constant Current Driver operates

as a fixed frequency, synchronous boost converter. The

switching frequency is internally maintained with a pre-

cision oscillator typically set to 1 MHz. Because the

LEDs require high currents, the device will work in

PWM Continuous mode. At very low LED currents, the

MCP1643 might run in PWM Discontinuous mode. As

it features an anti-ringing control, the switching noise is

low. The P-Channel switch acts as a synchronous rec-

tifier, by turning off to prevent reverse current flow from

the output cap back to the input in order to keep effi-

ciency high.

Lossless current sensing converts the peak current sig-

nal to a voltage to sum with the internal slope compen-

sation. This summed signal is compared to the voltage

error amplifier output to provide a peak current control

command

for

the

PWM

signal.

The

slope

compensation is adaptive to the input and output

voltage. Therefore, the converter provides the proper

amount of slope compensation to ensure stability, but is

not excessive, which causes a loss of phase margin.

The peak current limit is set to 1.6 A typical.

4.2.3

ADJUSTABLE OUTPUT LED

CURRENT

The MCP1643 LED’s current is adjustable with an

external resistor, called RSET, connected to VFB pin and

GND.

The device regulates the voltage on the RSET and pro-

vides a constant current trough LED while VIN  VOUT

(minus a 300 mV– 400 mV headroom in case of low

LED currents) (see Figures 2-1 and 2-2).

The internal VREF voltage is 120 mV. There are limits

applied when the RSET value is calculated over the

input voltages (see Typical Applications).

4.2.4

ENABLE

The enable pin is used to turn the boost converter on

and off. The enable threshold voltage varies with input

voltage. To enable the boost converter, the EN voltage

level must be greater than 75% of the VIN voltage. To

disable the boost converter, the EN voltage must be

less than 20% of the VIN voltage.

4.2.4.1

True Output Disconnect

The MCP1643 device incorporates a true output

disconnect feature. With the EN pin pulled low, the

output of the MCP1643 is isolated or disconnected

from the input by turning off the integrated P-Channel

switch and removing the switch bulk diode connection.

This removes the DC path, typical in boost converters,

which allows the output to be disconnected from the

input. During this mode, 1.2 µA (typical) of current is

consumed from the input (battery). True output discon-

nect does not discharge the output; this allows a faster

start-up in dimming or load step applications.

4.2.4.2

PWM Dimming

The MCP1643 allows dimming by turning the LED on

and off with a variable duty cycle PWM signal applied

to the EN pin. The maximum frequency for dimming is

limited by the internal soft-start of 240 µs (typical). By

varying the duty cycle of the PWM signal applied on EN

input, the LED current is changing linearly (see

Figure 2-8).

4.2.5

INTERNAL BIAS

The MCP1643 LED Constant Current Driver gets its

start-up bias from VIN. Once the output exceeds the

input, bias comes from the output. Therefore, once

started, the operation is completely independent of VIN.

The operation is only limited by the output power level

and the input source series resistance. Once started,

the output will remain in regulation, down to 0.5V typi-

cal with 25 mA LED current for low-source impedance

inputs.

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