MCP16252 datasheet(12/34 Pages) MICROCHIP

12 / 34 page

MCP16251/2

DS25173A-page 12

 2013 Microchip Technology Inc.

4.2.1

LOW-VOLTAGE START-UP

The MCP16251/2 is capable of starting from a low input

voltage. Start-up voltage is typically 0.82V for a 3.3V

output and 1 mA resistive load.

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. If the output

voltage is below 60-70% of the desired VOUT, the

device runs in open-loop with a fixed duty cycle of

70-75% until the output reaches this threshold. During

start-up, the inductor peak current is limited (see

Figure 2-21) to allow a correct start from a weak power

supply, such as a solar cell, small coin battery or a

discharged battery. Once the output voltage reaches

60-70% of the desired VOUT, normal closed-loop PWM

operation is initiated.

The MCP16251/2 charges an internal capacitor with a

very weak current source. The voltage on this capaci-

tor, in turn, slowly ramps the current limit of the boost

switch to its nominal value. The soft-start capacitor is

completely discharged in the event of a commanded

shutdown or a thermal shutdown.

There is no undervoltage lockout feature for the

MCP16251/2. The device will start switching at the

lowest voltage possible, and run down to the lowest

possible voltage. For a minimum 0.82V typical input,

the device starts with regulated output under 1 mA

resistive load. Real world loads are mostly non-

resistive and allow device start-up at lower values,

down to 0.65V. Working at very low input voltages may

result

“motor-boating”

for

deeply

discharged

batteries.

4.2.2

PFM/PWM MODE

The MCP16251/2 devices are capable of automatically

operating in normal PWM mode and PFM mode to

maintain high efficiency at all loads. In PFM mode, the

output ripple has a variable frequency component that

changes with the input voltage and output current. The

value of the output capacitor changes the low

frequency component ripple. Output ripple peak-to-

peak values are not affected by the output capacitor.

With no load, the input current drawn from the battery

is typically 14 µA. The device itself is powering from the

output after start-up, the quiescent current drawn from

output being less than 4 µA (typical, without feedback

resistors divider current).

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 MCP16251 enters smoothly

into the PWM mode.

In PWM operation, the MCP16251/2 operates as a

fixed frequency, synchronous boost converter. The

switching frequency is internally maintained with a pre-

cision oscillator, typically set to 500 kHz. By operating

in PWM-only mode, the output ripple remains low and

the frequency is constant.

Lossless current sensing converts the peak current

signal to a voltage to sum with the internal slope

compensation signal. 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 650 mA typical.

4.2.3

ADJUSTABLE OUTPUT VOLTAGE

AND MAXIMUM OUTPUT CURRENT

The MCP16251/2 output voltage is adjustable with a

resistor divider over a 1.8V minimum to 5.5V maximum

range. High value resistors are recommended to mini-

mize quiescent current to keep efficiency high at light

loads. When an application runs below -20oC, smaller

values for feedback resistors should be used to avoid

any alteration of VOUT, because of the leakage path on

PCBs.

The maximum device output current is dependent upon

the input and output voltage. For example, to ensure a

100 mA load current for VOUT = 3.3V, a minimum of

1.1 – 1.2V input voltage is necessary. If an application

is powered by one Li-Ion battery (VIN from 3.0V to

4.2V), the maximum load current the MCP16251/2 can

deliver is close to 200 mA at 5.0V output (Figure 2-10).

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 70% of the VIN voltage. To

disable the boost converter, the EN voltage must be

less than 20% of the VIN voltage.

4.2.5

INTERNAL BIAS

The MCP16251/2 gets its start-up bias from VIN. Once

the output exceeds the input, bias comes from the

output.

Therefore,

once

started,

operation

completely independent of VIN. Operation is limited

only by the output power level and the input source

series resistance. Once started, the output will remain

in regulation down to 0.35V input with 1 mA output

current for low source impedance inputs.

MCP16252 Datasheet(PDF) 12 Page - Microchip Technology

MCP16252 Datasheet(HTML) 12 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Datasheet Download

Link URL