Datasheet

 2009-2013 Microchip Technology Inc. DS22201B-page 13
MCP14700
5.0 APPLICATION INFORMATION
5.1 Bootstrap Capacitor Select
The selection of the bootstrap capacitor is based upon
the total gate charge of the high-side power MOSFET
and the allowable droop in gate drive voltage while the
high-side power MOSFET is conducting.
EQUATION 5-1:
For example:
Q
GATE
= 30 nC
V
DROOP
= 200 mV
C
BOOT
0.15 uF
A low ESR ceramic capacitor is recommend with a
maximum voltage rating that exceeds the maximum
input voltage, V
CC
, plus the maximum supply voltage,
V
SUPPLY
. It is also recommended that the capacitance
of C
BOOT
does not exceed 1.2 uF.
5.2 Decoupling Capacitor
Proper decoupling of the MCP14700 is highly
recommended to help ensure reliable operation. This
decoupling capacitor should be placed as close to the
MCP14700 as possible. The large currents required to
quickly charge the capacitive loads are provided by this
capacitor. A low ESR ceramic capacitor is
recommended.
5.3 Power Dissipation
The power dissipated in the MCP14700 consists of the
power loss associated with the quiescent power and
the gate charge power.
The quiescent power loss can be calculated by the
following equation and is typically negligible compared
to the gate drive power loss.
EQUATION 5-2:
The main power loss occurs from the gate charge
power loss. This power loss can be defined in terms of
both the high-side and low-side power MOSFETs.
EQUATION 5-3:
C
BOOT
Q
GATE
V

DROO P
-----------------------------

Where:
C
BOOT
= Bootstrap capacitor value
Q
GATE
= Total gate charge of the high-side
MOSFET
V
DROO
= Allowable gate drive voltage droop
P
Q
I
VCC
V
CC
 =
Where:
P
Q
= Quiescent power loss
I
VCC
= No Load Bias Current
V
CC
= Bias Voltage
P
GATE
P
HIGHDR
P
LOWDR
+=
P
HIGHDR
V
CC
Q
HIGH
 F
SW
 =
P
LOWDR
V
CC
Q
LOW
 F
SW
 =
Where:
P
GATE
= Total Gate Charge Power Loss
P
HIGHDR
= High-Side Gate Charge Power Loss
P
LOWDR
= Low-Side Gate Charge Power Loss
V
CC
= Bias Supply Voltage
Q
HIGH
= High-Side MOSFET Total Gate
Charge
Q
LOW
= Low-Side MOSFET Total GAte
Charge
F
SW
= Switching Frequency