2004-2013 Microchip Technology Inc. DS21876B-page 13
MCP1650/51/52/53
4.0 DETAILED DESCRIPTION
4.1 Device Overview
The MCP1650/51/52/53 is a gated oscillator boost
controller. By adding an external N-channel MOSFET,
schottky diode and boost inductor, high-output power
applications can be achieved. The 750 kHz hysteretic
gated oscillator architecture enables the use of small,
low-cost external components. By using a hysteretic
approach, no compensation components are
necessary for the stability of the regulator output.
Output voltage regulation is accomplished by
comparing the output voltage (sensed through an
external resistor divider) to a reference internal to the
MCP1650/51/52/53. When the sensed output voltage
is below the reference, the EXT pin pulses the external
N-channel MOSFET on and off at the 750 kHz gated
oscillator frequency. Energy is stored in the boost
inductor when the external N-channel MOSFET is on
and is delivered to the load through the external
Schottky diode when the MOSFET is turned off.
Several pulses may be required to deliver enough
energy to pump the output voltage above the upper
hysteretic limit. Once above the hysteretic limit, the
internal oscillator is no longer gated to the EXT pin and
no energy is transferred from input to output.
The peak current in the MOSFET is sensed to limit its
maximum value. As with all boost topology converters,
even though the MOSFET is turned off, there is still a
DC path through the boost inductor and diode to the
load. Additional protection circuity, such as fuses, are
recommended for short circuit protection.
4.2 Input Voltage
The range of input voltage for the MCP1650/51/52/53
family of devices is specified from 2.7V to 5.5V. For the
S-option devices, the undervoltage lockout (UVLO)
feature will turn the boost controller off once the input
voltage falls below 2.55V, typical. For the R-option
devices, the UVLO is set to 2.0V. The R-option devices
are recommended for use when “bootstrapping” the
output voltage back to the input. The input of the
MCP1650/51/52/53 device is supplied by the output
voltage during boost operation. This can be used to
derive output voltages from input voltages that start up
at approximately 2V (2-cell alkaline batteries).
4.3 Fixed Duty Cycle
The MCP1650/51/52/53 family utilizes a unique two-
step maximum duty cycle architecture to minimize input
peak current and improve output ripple voltage for wide
input voltage operating ranges. When the input voltage
is below 3.8V, the duty cycle is typically 80%. For input
voltages above 3.8V, the duty cycle is typically 56%. By
decreasing the duty cycle at higher input voltages, the
input peak current is reduced. For low input voltages, a
longer duty cycle stores more energy during the on-
time of the boost MOSFET. For applications that span
the 3.8V input range, the inductor value should be
selected to meet not only the minimum input voltage at
80% duty cycle, but 3.8V at 56% duty cycle as well.
Refer to Section 5.0 “Application Circuits/Issues”
for more information about selecting inductor values.
4.4 Shutdown Input Operation
The SHDN pin is used to turn the MCP1650/51/52/53
on and off. When the SHDN
pin is tied low, the
MCP1650/51/52/53 is off. When tied high, the
MCP1650/51/52/53 will be enabled and begin boost
operation as long as the input voltage is not below the
UVLO threshold.
4.5 Soft-Start Operation
When power is first applied to the MCP1650/51/52/53,
the internal reference initialization is controlled to slow
down the start-up of the boost output voltage.This is
done to reduce high inrush current required from the
source. High inrush currents can cause the source
voltage to drop suddenly and trip the UVLO threshold,
shutting down the converter prior to it reaching steady-
state operation.
4.6 Gated Oscillator Architecture
A 750 kHz internal oscillator is used as the base
frequency of the MCP1650/51/52/53. The oscillator
duty cycle is typically 80% when the input voltage is
below a nominal value of 3.8V, and 56% when the
input voltage is above a nominal value of 3.8V. Two
duty cycles are provided to reduce the peak inductor
current in applications where the input voltage varies
over a wide range. High-peak inductor current results
in undesirable high-output ripple voltages. For
applications that have input voltage that cross this
3.8V boundary, both duty cycle conditions need to be
examined to determine which one has the least
amount of energy storage. Refer to Section 5.0
“Application Circuits/Issues” for more information
about design considerations.
