Microchip Technology ADM00423 Data Sheet
© 2011 Microchip Technology Inc.
DS22285A-page 17
MCP16321/2
4.2
Functional Description
4.2.1
STEP-DOWN OR BUCK
CONVERTER
CONVERTER
The MCP16321/2 is a synchronous, step-down or buck
converter capable of stepping input voltages ranging
from 6V to 24V 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 24V 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.
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