MCP1661 datasheet(13/34 Pages) MICROCHIP

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DS20005315B-page 13

MCP1661

4.2.1

INTERNAL BIAS

The MCP1661 device gets its bias from VIN. The VIN

bias is used to power the device and drive circuits over

the entire operating range.

4.2.2

START-UP VOLTAGE

AND SOFT START

The MCP1661 device starts at input voltages that are

higher than or equal to a predefined set UVLO value.

MCP1661 starts switching at approximately 2.3V for

12.0V output and 1 mA resistive load. Once started, the

device will continue to operate under normal load

conditions down to 1.85V typical. There is a soft start

feature which provides a way to limit the inrush current

drawn from the input (batteries) during start-up. The

soft start has an important role in applications where

the switch will reach 32V. During start-up, excessively

high switch current, together with the presence of high

voltage, can overstress the NMOS switch.

When the device is powered (EN = VIN and VIN rises

from zero to its nominal value), the output capacitor

charges to a value close to the input voltage (or VIN

minus a Schottky diode voltage drop). The overshoot

on output is limited by slowly increasing the reference

of the error amplifier. There is an internal reference

voltage which charges an internal capacitor with a

weak current source. The voltage on this capacitor

slowly ramps the reference voltage. The soft-start

capacitor is completely discharged in the event of a

commanded shutdown or a thermal shutdown.

Due to the direct path from input to output, in the case

of start-up by enable (EN voltage switches from low-to-

high), the output capacitor is already charged and the

output starts from a value close to the input voltage.

The internal oscillator has a delayed start to let the

output capacitor be completely charged to the input

voltage value.

4.2.3

UNDERVOLTAGE LOCKOUT

(UVLO)

MCP1661 features an UVLO which prevents fault

operation below 1.85V, which corresponds to the

typical value of two discharged batteries. The device

starts its normal operation at 2.3V input. The upper limit

is set to avoid any input transients (temporary VIN

drop), which might trigger the lower UVLO threshold

and restart the device. Usually, these voltage transients

(overshoots and undershoots) have up to a few

hundred mV.

MCP1661 is a non-synchronous boost regulator. Due

to this fact, there is a direct path from VIN to VOUT

through the inductor and the diode. This means that,

while the device is not switching (VIN below UVLOSTOP

threshold), VOUT is not zero but equal to VIN –VF

(where VF is the voltage drop on the rectifier diode).

When the input voltage is below the 2.3V UVLO start

threshold, the device is operating with limited

specification.

4.2.4

PWM MODE OPERATION

MCP1661

operates

fixed-frequency,

non-synchronous converter. The switching frequency

is maintained at 500 kHz with a precision oscillator.

Lossless current sensing converts the peak current

signal to a voltage (VSENSE) and adds it to the internal

slope compensation (VRAMP). This summed signal is

compared to the voltage error amplifier output (VERROR)

to provide a peak current control signal (VPWM) for the

PWM control block. The slope compensation signal

depends on the input voltage. Therefore, the converter

provides the proper amount of slope compensation to

ensure stability. The peak current is set to 1.3A.

The MCP1661 device will operate in PWM even during

periods of light load operation by skipping pulses. By

operating in PWM mode, the output ripple is low and

the frequency is constant.

4.2.5

ADJUSTABLE OUTPUT VOLTAGE

The MCP1661 output voltage is adjustable with a

resistor divider over the VOUT range. High value

resistors are recommended to minimize power loss and

keep efficiency high at light loads. The device

integrates a transconductance-type error amplifier and

the values of the feedback resistors do not influence

the stability of the system.

4.2.6

MINIMUM INPUT VOLTAGE

AND MAXIMUM OUTPUT CURRENT

The maximum output current for which the device can

supply the load is dependent upon the input and output

voltage. The minimum input voltage necessary to reach

the value of the desired output depends on the

maximum

duty

cycle

(approximately

90%)

accordance

with

the

mathematical

relation

VOUT =VINmin/(1 – DMAX). As there is a 1.3A inductor

peak current limit, VOUT can go out of regulation before

reaching the maximum duty cycle. (For boost

converters, the average inductor current is equal to the

input current.)

For example, to ensure a 100 mA load current for

VOUT = 12.0V, a minimum of 2.8V input voltage is

necessary. If an application is powered by one Li-Ion

battery (VIN from 3.3V to 4.2V), the minimum load

current the MCP1661 device can deliver is close to

50 mA at 24.0V output (see Figure 2-3).

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