Datasheet

ICLoss_driver IN g_total s
P V Q f= × ×
2
bottom CHG DS(on)
P = (1 D) I R- ´ ´
REG N plt plt
on off
on off
V V V
I = , I =
R R
-
SW GD GS
1
Q = Q + Q
2
´
SW SW
on off
on off
Q Q
t = , t =
I I
( )
2
top CHG DS(on) IN CHG on off S
1
P = D I R + V I t + t f
2
´ ´ ´ ´ ´ ´
top DS(on) GD bottom DS(on) G
FOM = R Q FOM = R Q´ ´
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SLUS892B –DECEMBER 2009–REVISED SEPTEMBER 2013 www.ti.com
Power MOSFETs 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 prefered 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 the 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
.
(17)
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
CHG
), the MOSFET on-resistance t
DS(on)
), input voltage (V
IN
), switching frequency
(f
S
), turnon time (t
on
) and turnoff time (t
off
):
(18)
The first item represents the conduction loss. Usually MOSFET r
DS(on)
increases by 50% with 100ºC junction
temperature rise. The second term represents the switching loss. The MOSFET turnon and turnoff times are
given by:
(19)
where Q
sw
is the switching charge, I
on
is the turnon gate-driving current and I
off
is the turnoff gate driving 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
):
(20)
Total gate-driving current can be estimated by the REGN voltage (V
REGN
), MOSFET plateau voltage (V
plt
), total
turnon gate resistance (R
on
), and turnoff gate resistance (R
off
) of the gate driver:
(21)
The conduction loss of the bottom-side MOSFET is calculated with the following equation when it operates in
synchronous continuous-conduction mode:
(22)
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 nonsynchronous-mode charging current.
MOSFET gate-driver power loss contributes to the dominant losses on the controller IC when the buck converter
is switching. Choosing the MOSFET with a small Q
g_total
reduces the IC power loss to avoid thermal shutdown.
(23)
where Q
g_total
is the total gate charge for both upper and lower MOSFETs at 6-V V
REGN
.
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