Datasheet

MCP16331
DS20005308C-page 14  2014-2016 Microchip Technology Inc.
FIGURE 4-2: Step-Down Converter.
4.2.2 PEAK CURRENT MODE CONTROL
The MCP16331 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 4-2.
4.2.3 PULSE-WIDTH MODULATION (PWM)
The internal oscillator periodically starts the switching
period, which in the MCP16331 device’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 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
error amplifier output rises. This results in an increase in
the inductor current to correct for error in the output volt
-
age. 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 set 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 terminate the cycle.
When working close to the boundary conduction
threshold, a jitter on the SW node may occur, reflecting
in the output voltage. Although the low-frequency
output component is very small, it may be desirable to
completely eliminate this component. To achieve this,
different methods can be applied to reduce or
completely eliminate this component. In addition to a
very good layout, a capacitor in parallel with the top
feedback resistor, or an RC snubber between the SW
node and GND, can be added.
Typical values for the snubber are 680 pF and 430,
while the capacitor in parallel with the top feedback
resistor can use values from 10 pF to 47 pF. Using such
a snubber eliminates the ringing on the SW node, but
decreases the overall efficiency of the converter.
4.2.4 HIGH-SIDE DRIVE
The MCP16331 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). 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 induc
-
tor and Schottky diode or switch node. When the switch
is off, the boost cap voltage is replenished, typically
from the output voltage for 3V to 5V output applica
-
tions. 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 start-up, the boost cap has no stored charge to
drive the switch. An internal regulator is used to “pre
-
charge” the boost cap. Once precharged, the switch is
turned on and the inductor current flows. When the
switch turns off, the inductor current freewheels 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
precharge current to replace the boost cap charge. For
input voltages above 5.5V typical, the MCP16331 device
will regulate the output voltage with no load. After start
-
ing, the MCP16331 will regulate the output voltage until
the input voltage decreases below 4V. See
Figure 2-23
for device range of operation over input voltage, output
voltage and load.
Schottky
Diode
C
OUT
V
OUT
SW
V
IN
+
–
SW
on off on on
off
I
L
I
L
L
I
OUT
V
OUT
V
IN
0
SW
on off on on
off
I
L
I
OUT
V
IN
0
Continuous Inductor Current Mode
Discontinuous Inductor Current Mode