Datasheet
DocID15086 Rev 3 13/20
PM8834 Design guidelines
20
Equation 1
• The power of the driver is defined as the power needed by the driver to continuously
switch ON and OFF the external MOSFETs; it is a function of the switching frequency
and total gate charge of the selected MOSFETs. It can be quantified considering that
the total power P
SW
dissipated to switch the MOSFETs is dissipated by three main
factors: external gate resistance (when present), intrinsic MOSFET resistance and
intrinsic driver resistance. This last term has to be determined to calculate the device
power dissipation.The total power dissipated by each section to switch an external
MOSFETs with gate charge Q
G
is:
Equation 2
When designing an application based on the PM8834 it is recommended to take into
consideration the effect of the external gate resistors on the power dissipated by the driver.
External gate resistors help the device to dissipate the switching power since the same
power P
SW
will be shared between the internal driver impedance and the external resistor,
resulting in a general cooling of the device.
Referring to Figure 6, a typical MOSFET driver can be represented by a push-pull output
stage with two different MOSFETs: P-DMOS to drive the external gate high and N-DMOS to
drive the external gate low (with their own Rds
ON
: R
hi
, R
lo
). The external power MOSFET
can be represented in this case as a capacitance (C
G
) that stores the gate-charge (Q
G
)
required by the external power MOSFET to reach the driving voltage (V
CC
). This
capacitance is charged and discharged at the driver switching frequency F
SW
.The total
power P
SW
is dissipated among the resistive components distributed along the driving path.
According to the external gate resistance and the power MOSFET intrinsic gate resistance,
the driver dissipates only a portion of P
SW
(per section) as follows:
Equation 3
The total power dissipated from the driver can then be determined as follows:
Equation 4
P
DC
V
CC
I
CC
⋅=
P
SW
F
SW
Q
G
V
CC
⋅()⋅=
P
SW
1
2
---
C
G
V
CC
()
2
F
SW
R
hi
R
hi
R
Gate
R
i
++
-----------------------------------------
R
Io
R
Io
R
Gate
R
i
++
----------------------------------------- -+
⋅⋅ ⋅ ⋅=
PP
DC
2P
SW
⋅+=