Datasheet
2011-2012 Microchip Technology Inc. DS25004B-page 13
MCP16301
4.2.2 PEAK CURRENT MODE CONTROL
The MCP16301 integrates a Peak Current Mode
Control architecture, resulting in superior AC regulation
while minimizing the number of voltage loop
compensation components, and their size, for
integration. Peak Current Mode Control takes a small
portion of the inductor current, replicates it and
compares this replicated current sense signal with the
output of the integrated error voltage. In practice, the
inductor current and the internal switch current are
equal during the switch-on time. By adding this peak
current sense to the system control, the step-down
power train system is reduced from a 2
nd
order to a 1
st
order. This reduces the system complexity and
increases its dynamic performance.
For Pulse-Width Modulation (PWM) duty cycles that
exceed 50%, the control system can become bimodal
where a wide pulse followed by a short pulse repeats
instead of the desired fixed pulse width. To prevent this
mode of operation, an internal compensating ramp is
summed into the current shown in
Figure 4-1.
4.2.3 PULSE-WIDTH MODULATION
(PWM)
The internal oscillator periodically starts the switching
period, which in MCP16301’s case occurs every 2
µs
or 500
kHz. With the integrated switch turned on, the
inductor current ramps up until the sum of the current
sense and slope compensation ramp exceeds the inte-
grated error amplifier output. The error amplifier output
slews up or down to increase or decrease the inductor
peak current feeding into the output LC filter. If the reg-
ulated output voltage is lower than its target, the invert-
ing error amplifier output rises. This results in an
increase in the inductor current to correct for errors in
the output voltage.
The fixed frequency duty cycle is terminated when the
sensed inductor peak current, summed with the inter
-
nal slope compensation, exceeds the output voltage of
the error amplifier. The PWM latch is reset, by turning
off the internal switch and preventing it from turning on
until the beginning of the next cycle. An overtempera
-
ture signal, or boost cap undervoltage, can also reset
the PWM latch to asynchronously terminate the cycle.
4.2.4 HIGH SIDE DRIVE
The MCP16301 features an integrated high-side
N-Channel MOSFET for high efficiency step-down
power conversion. An N-Channel MOSFET is used for
its low resistance and size (instead of a P-Channel
MOSFET). The N-Channel MOSFET gate must be
driven above its source to fully turn on the transistor. A
gate-drive voltage above the input is necessary to turn
on the high side N-Channel. The high side drive voltage
should be between 3.0V and 5.5V. The N-Channel
source is connected to the inductor and Schottky diode,
or switch node.
When the switch is off, the inductor current flows
through the Schottky diode, providing a path to
recharge the boost cap from the boost voltage source,
typically the output voltage for 3.0V to 5.0V output
applications. A boost-blocking diode is used to prevent
current flow from the boost cap back into the output
during the internal switch-on time. Prior to startup, the
boost cap has no stored charge to drive the switch. An
internal regulator is used to “pre-charge” the boost cap.
Once pre-charged, the switch is turned on and the
inductor current flows. When the switch turns off, the
inductor current free-wheels through the Schottky
diode, providing a path to recharge the boost cap.
Worst case conditions for recharge occur when the
switch turns on for a very short duty cycle at light load,
limiting the inductor current ramp. In this case, there is
a small amount of time for the boost capacitor to
recharge. For high input voltages there is enough pre-
charge current to replace the boost cap charge. For
input voltages above 5.5V typical, the MCP16301
device will regulate the output voltage with no load.
After starting, the MCP16301 will regulate the output
voltage until the input voltage decreases below 4V. See
Figure 2-16 for device range of operation over input
voltage, output voltage and load.
4.2.5 ALTERNATIVE BOOST BIAS
For 3.0V to 5.0V output voltage applications, the boost
supply is typically the output voltage. For applications
with 3.0V
< V
OUT
< 5.0V, an alternative boost supply
can be used.
Alternative boost supplies can be from the input, input
derived, output derived or an auxiliary system voltage.
For low voltage output applications with unregulated
input voltage, a shunt regulator derived from the input
can be used to derive the boost supply. For
applications with high output voltage or regulated high
input voltage, a series regulator can be used to derive
the boost supply.