Microchip Technology ADM00427 Data Sheet
© 2011 Microchip Technology Inc.
DS22284A-page 17
MCP16323
4.2
Functional Description
4.2.1
STEP-DOWN OR BUCK
CONVERTER
CONVERTER
The MCP16323 is a synchronous, step-down or buck
converter capable of stepping input voltages ranging
from 6V to 18V down to 0.9V to 5V.
The integrated high-side switch is used to chop or
modulate the input voltage using a controlled duty cycle
for output voltage regulation. The integrated low-side
switch is used to freewheel current when the high-side
switch is turned off. High efficiency is achieved by using
low-resistance switches and low equivalent series
resistance (ESR), inductor and capacitors. When the
high-side switch is turned on, a DC voltage is applied to
the inductor (V
converter capable of stepping input voltages ranging
from 6V to 18V down to 0.9V to 5V.
The integrated high-side switch is used to chop or
modulate the input voltage using a controlled duty cycle
for output voltage regulation. The integrated low-side
switch is used to freewheel current when the high-side
switch is turned off. High efficiency is achieved by using
low-resistance switches and low equivalent series
resistance (ESR), inductor and capacitors. When the
high-side switch is turned on, a DC voltage is applied to
the inductor (V
IN
– V
OUT
), resulting in a positive linear
ramp of inductor current. When the high-side switch
turns off and the low-side switch turns on, the applied
inductor voltage is equal to –V
turns off and the low-side switch turns on, the applied
inductor voltage is equal to –V
OUT
, resulting in a
negative linear ramp of inductor current. In order to
ensure there is no shoot through current, a dead time
where both switches are off is implemented between
the high-side switch turning off and the low-side switch
turning on, and the low-side switch turning off and the
high-side switch turning on.
For steady-state, continuous inductor current
operation, the positive inductor current ramp must
equal the negative current ramp in magnitude. While
operating in steady state, the switch duty cycle must be
equal to the relationship of V
ensure there is no shoot through current, a dead time
where both switches are off is implemented between
the high-side switch turning off and the low-side switch
turning on, and the low-side switch turning off and the
high-side switch turning on.
For steady-state, continuous inductor current
operation, the positive inductor current ramp must
equal the negative current ramp in magnitude. While
operating in steady state, the switch duty cycle must be
equal to the relationship of V
OUT
/V
IN
for constant
output voltage regulation, under the condition that the
inductor current is continuous, or never reaches zero.
For discontinuous inductor current operation, the
steady-state duty cycle will be less than V
inductor current is continuous, or never reaches zero.
For discontinuous inductor current operation, the
steady-state duty cycle will be less than V
OUT
/V
IN
to
maintain voltage regulation. When the inductor current
reaches zero, the low-side switch is turned off so that
current does not flow in the reverse direction, keeping
the efficiency high. The average of the chopped input
voltage or SW node voltage is equal to the output
voltage, while the average inductor current is equal to
the output current.
reaches zero, the low-side switch is turned off so that
current does not flow in the reverse direction, keeping
the efficiency high. The average of the chopped input
voltage or SW node voltage is equal to the output
voltage, while the average inductor current is equal to
the output current.
FIGURE 4-2:
Synchronous Step-Down
Converter.
4.2.2
PEAK CURRENT MODE CONTROL
The MCP16323 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 can be approximated by a 1
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 can be approximated by a 1
st
order
system rather than a 2
nd
order system. This reduces
the system complexity and increases its dynamic
performance.
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 sense signal.
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 sense signal.
4.2.3
PULSE WIDTH MODULATION
(PWM)
(PWM)
The internal oscillator periodically starts the switching
period, which in the MCP16323’s case occurs every
1 µs or 1 MHz. With the high-side integrated
N-Channel MOSFET turned on, the inductor current
ramps up until the sum of the current sense and slope
compensation ramp exceeds the integrated error
amplifier output. Once this occurs, the high-side switch
period, which in the MCP16323’s case occurs every
1 µs or 1 MHz. With the high-side integrated
N-Channel MOSFET turned on, the inductor current
ramps up until the sum of the current sense and slope
compensation ramp exceeds the integrated error
amplifier output. Once this occurs, the high-side switch
SW
I
L
V
IN
I
OUT
V
OUT
Continuous Inductor Current Mode
S
1
ON
S
2
ON
SW
I
L
V
IN
I
OUT
Discontinuous Inductor Current Mode
S
1
ON S
2
ON
Both
OFF
OFF
V
IN
L
I
L
C
OUT
V
OUT
S
2
S
1