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MCP16251 Datasheet(PDF) 12 Page - Microchip Technology

Part # MCP16251
Description  Low Quiescent Current, PFM/PWM Synchronous Boost Regulator with True Output Disconnect or Input/Output Bypass Option
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Manufacturer  MICROCHIP [Microchip Technology]
Direct Link  http://www.microchip.com
Logo MICROCHIP - Microchip Technology

MCP16251 Datasheet(HTML) 12 Page - Microchip Technology

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MCP16251/2
DS20005173B-page 12
 2013 - 2016 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
in
“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 once or several times to bring the output back
into regulation. If the output load current rises above
the upper threshold, the MCP16251/2 enters smoothly
into 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 -20°C, 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 on
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 (refer to
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
lower 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
is
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.


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