MCP1641X datasheet(22/44 Pages) MICROCHIP

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MCP1641X

DS20006394D-page 22

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5.3.1

INTERNAL BIAS

The MCP1641X devices get their start-up bias from

VIN. The VIN bias is used to power the device and

drive circuits over the entire operating range. During

normal operation, the internal VMAX comparator

selects the highest voltage rail between VIN and VOUT,

in order to optimize operation and reduce power

consumption. Once the output exceeds the input, bias

comes from the output. The internal voltage reference

of 485 mV is powered from the input voltage at all

times. A voltage amplifier buffers and multiplies the

reference to 0.97V for the FB input of the error

amplifier. Once the UVLO comparator triggers the

start-up, the internal control loop keeps the output in

regulation,

while

VIN ramps down to 0.8V

(UVLOSTOP).

5.3.2

LOW-VOLTAGE START-UP

The MCP1641X is capable of starting from a low input

voltage. Start-up voltage is well-controlled by the UVLO

circuitry, which uses the 485 mV voltage reference.

The default start-up value is 0.85V (typical). The

UVLOSTART threshold can be programmed by using an

external resistive divider connected to the UVLO pin.

This input also serves as a low battery input.

When the device is enabled (EN set high) and the input

voltage is higher than 0.85V (typical), the internal

start-up logic turns on the rectifying P-Channel switch

until the output capacitor is charged to a value close to

the input voltage. This is commonly called the output

precharging phase and the rectifying switch limits the

current during this time. Precharge current varies and

increases with VIN. Precharge current starts from

25 mA for low input voltage and increases up to

250 mA or more near the maximum limit of VIN.

After the output capacitor is charged to the input

voltage, the device starts switching and runs in open

loop,

with

limited

inductor

peak

current,

approximately 30-40% of its nominal value. Once the

output voltage ramps up to 60-70% of the nominal

value, the normal closed-loop operation is initiated.

5.3.3

UNDERVOLTAGE LOCKOUT (UVLO)

The internal UVLO comparator input uses the 485 mV

voltage reference to compare it with the battery input

voltage. If the UVLO input is tied to VIN, the comparator

enables the converter at 0.85V typical input voltage. If

a different UVLOSTART voltage is desired, a resistive

divider must be connected to the UVLO pin.

The UVLOSTOP threshold is set internally to 0.8V.

5.3.4

PFM/PWM OPERATION

The MCP16411/3/5/7 devices use an automatic

switchover from PWM to PFM mode, for light load

conditions, to maximize efficiency over a wide range of

output current. During PFM mode, a controlled peak

current limit is used to pump the output up to the

threshold. While operating in PFM or PWM mode, the

P-Channel switch is used as a synchronous rectifier,

turning off when the inductor current reaches 0 mA, in

order to maximize efficiency.

In PFM mode, a voltage comparator is used to terminate

switching when the output voltage reaches an upper

threshold limit. Once switching has terminated, the out-

put voltage decays or coasts down. During this Sleep

period, a very low current is consumed from the input

source, which keeps power efficiency high at light load.

The PFM mode frequency is a function of input voltage,

output voltage and load. While in PFM mode, the boost

converter periodically pumps the output with a fixed

switching frequency of 500 kHz. The value of the

output

capacitor

changes

the

low-frequency

component ripple. The device itself is powered from the

output and consumes 5 µA (typical).

PFM operation is initiated if the output load current falls

below an internally programmed threshold. The output

voltage is continuously monitored; when the output

voltage drops below its nominal value, PFM operation

pulses one or several times to bring the output back

into regulation. If the output load current rises above

the upper threshold, the MCP16411/3/5/7 enters

smoothly into the PWM mode.

Figure 2-12 represents the input voltage versus load

current for the PFM to PWM threshold.

5.3.5

PWM ONLY OPERATION

In normal PWM Operation mode, the MCP16412/4/6/8

devices operate as a fixed frequency, synchronous

boost converter. The switching frequency is internally

maintained with a precision oscillator, which is typically

set to 500 kHz.

At light loads, the MCP16412/4/6/8 devices begin to

skip pulses. By operating in PWM Only mode, the

output ripple remains low and the frequency is

constant. Operating in Fixed PWM mode results in low

efficiency during light load operation, but with the

advantage of low output ripple and noise for the

supplied load. Lossless current sensing converts the

peak current signal to voltage in order to sum it with the

internal slope compensation. This summed signal is

compared with the voltage error amplifier output to

provide a peak current control command for the PWM

signal. The converter provides the proper amount of

slope compensation to ensure stability. The peak

current limit is typically set to 1A.

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