Datasheet

top DS(on) G D bottom DS(on) G
FOM = R Q FOM = R Q´ ´
( )
2
top CHG DS(on) IN CHG on off S
1
P = D I R + V I t + t f
2
´ ´ ´ ´ ´ ´
SW SW
on off
on off
Q Q
t = , t =
I I
SW GD GS
1
Q = Q + Q
2
´
REG N plt plt
on off
on off
V V V
I = , I =
R R
-
2
bottom CHG DS(on)
P = (1 D) I R- ´ ´
ICLoss_driver IN g_total s
P V Q f= × ×
bq24600
SLUS891A –FEBRUARY 2010– REVISED OCTOBER 2011 www.ti.com
Power MOSFET Selection
Two external N-channel MOSFETs are used for a synchronous switching battery charger. The gate drivers are
internally integrated into the IC with 6 V of gate drive voltage. 30-V or higher voltage rating MOSFETs are
preferred for 20-V input voltage, and 40-V or higher rating MOSFETs are preferred for 20-V to 28-V input
voltage.
Figure-of-merit (FOM) is usually used for selecting the proper MOSFET based on a tradeoff between the
conduction loss and switching loss. For a top-side MOSFET, FOM is defined as the product of a MOSFET
on-resistance, r
DS(on)
, and the gate-to-drain charge, Q
GD
. For a bottom-side MOSFET, FOM is defined as the
product of the MOSFET on-resistance, r
DS(on)
, and the total gate charge, Q
G
.
(16)
The lower the FOM value, the lower the total power loss. Usually lower r
DS(on)
has higher cost with the same
package size.
The top-side MOSFET loss includes conduction loss and switching loss. It is a function of duty cycle (D =
V
OUT
/V
IN
), charging current (I
CHARGE
), the MOSFET on-resistance r
DS(on)
), input voltage (V
IN
), switching frequency
(f
S
), turnon time (t
on
) and turnoff time (t
toff
):
(17)
The first item represents the conduction loss. Usually, MOSFET r
DS(on)
increases by 50% with a 100ºC
junction-temperature rise. The second term represents the switching loss. The MOSFET turnon and turnoff times
are given by:
(18)
where Q
sw
is the switching charge, I
on
is the turnon gate-drive current and I
off
is the turnoff gate-drive current. If
the switching charge is not given in the MOSFET data sheet, it can be estimated by gate-to-drain charge (Q
GD
)
and gate-to-source charge (Q
GS
):
(19)
Total gate-drive current can be estimated by REGN voltage (V
REGN
), MOSFET plateau voltage (V
plt
), total turn-on
gate resistance (Ron) and turn-off gate resistance R
off
) of the gate driver:
(20)
The conduction loss of the bottom-side MOSFET is calculated with the following equation when it operates in
synchronous continuous-conduction mode:
(21)
If the SRP-SRN voltage decreases below 5 mV (the charger is also forced into non-synchronous mode when the
average SRP-SRN voltage is lower than 1.25 mV), the low side FET is turned off for the remainder of the
switching cycle to prevent negative inductor current.
As a result, all the freewheeling current goes through the body diode of the bottom-side MOSFET. The maximum
charging current in non-synchronous mode can be up to 0.9 A (0.5 A typ.) for a 10-mΩ charging-current sensing
resistor considering IC tolerance. Choose the bottom-side MOSFET with either an internal Schottky or body
diode capable of carrying the maximum non-synchronous-mode charging current.
MOSFET gate driver power loss contributes to the dominant losses on the controller IC when the buck converter
is switching. Choosing a MOSFET with a small Q
g_total
largely reduces the IC power loss to avoid thermal
shutdown.
(22)
where Q
g_total
is the total gate charge for both upper and lower MOSFETs at 6-V V
REGN
.
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