Microchip Technology ADM00352 데이터 시트
2011-2013 Microchip Technology Inc.
DS20005004C-page 13
MCP16301/H
FIGURE 4-2:
Step-Down Converter.
4.2.2
PEAK CURRENT MODE CONTROL
The MCP16301/H devices integrate 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 to 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
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 to 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.
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
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
.
4.2.3
PULSE-WIDTH MODULATION
(PWM)
(PWM)
The internal oscillator periodically starts the switching
period, which, for MCP16301, 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
integrated 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 regulated output voltage is lower than its target, the
inverting error amplifier output rises. This results in an
increase in the inductor current to correct the errors in
the output voltage.
period, which, for MCP16301, 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
integrated 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 regulated output voltage is lower than its target, the
inverting error amplifier output rises. This results in an
increase in the inductor current to correct the errors in
the output voltage.
The fixed-frequency duty cycle is terminated when the
sensed inductor peak current, summed with the
internal 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
overtemperature signal, or boost cap undervoltage,
can also reset the PWM latch to asynchronously
terminate the cycle.
sensed inductor peak current, summed with the
internal 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
overtemperature signal, or boost cap undervoltage,
can also reset the PWM latch to asynchronously
terminate the cycle.
4.2.4
HIGH-SIDE DRIVE
The MCP16301/H devices feature 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.
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 precharge the boost cap.
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 precharge the boost cap.
Once precharged, 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/H
devices will regulate the output voltage with no load.
After starting, the MCP16301/H devices will regulate
the output voltage until the input voltage decreases
below 4V. See
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/H
devices will regulate the output voltage with no load.
After starting, the MCP16301/H devices will regulate
the output voltage until the input voltage decreases
below 4V. See
for device range of opera-
tion 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
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.
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.
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.