BQ24616 Datasheet, PDF (27/34 Pages) Texas Instruments – JEITA Guideline Compatible Stand-Alone Synchronous Switch-Mode Li-Ion or Li-Polymer Battery Charger with System Power Selector and Low Iq

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SLUSA49A – APRIL 2010 – REVISED MAY 2010
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 6V of gate drive voltage. 30V or higher voltage rating MOSFETs are
preferred for 20V input voltage and 40V or higher rating MOSFETs are perfered for 20-28V input voltage.
Figure-of-merit (FOM) is usually used for selecting proper MOSFET based on a tradeoff between the conduction
loss and switching loss. For top side MOSFET, FOM is defined as the product of a MOSFET's on-resistance,
R_DS(ON), and the gate-to-drain charge, Q_GD. For bottom side MOSFET, FOM is defined as the product of the
MOSFET's on-resistance, R_DS(ON), and the total gate charge, Q_G.
FOM_top = R_DS(on) ´ Q_{G D}
FOM_bottom = R_DS(on) ´ 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), MOSFET's on-resistance R_DS(ON)), input voltage (V_IN), switching frequency
(F), turn on time (t_on) and turn off time (t_toff):
( ) P_top = D ´ I_CHG2
´ R_DS(on) +
1
2
´ V_IN
´ I_CHG
´
t_on + t_off
´ f_S
(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 turn-on and turn off times are
given by:
t_on
=
Q_SW
I_on
,
t _off
=
Q_SW
I_off
(19)
where Q_sw is the switching charge, I_on is the turn-on gate driving current and Ioff is the turn-off gate driving
current. If the switching charge is not given in MOSFET datasheet, it can be estimated by gate-to-drain charge
(Q_GD) and gate-to-source charge (Q_GS):
1
Q_SW = Q_GD + ₂ ´ Q_GS
(20)
Gate driving current total 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:
I_on =
V_{REG N} -
R_on
V_{plt , Ioff}
=
V_plt
R_off
(21)
The conduction loss of the bottom-side MOSFET is calculated with the following equation when it operates in
synchronous continuous conduction mode:
P_bottom = (1 - D) ´ I_CHG ² ´ R_DS(on)
(22)
If the SRP-SRN voltage decreases below 5mV (The charger is also forced into non-synchronous mode when the
average SRP-SRN voltage is lower than 1.25mV), the low side FET will be 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 0.9A (0.5A typ) for a 10mΩ 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 controller IC, when the buck converter is
switching. Choosing the MOSFET with a small Q_{g_total} will reduce the IC power loss to avoid thermal shutdown.
P_{ICLoss_driver} = V_IN × Q_{g_total} × f_s
(23)
Where Q_{g_total} is the total gate charge for both upper and lower MOSFET at 6V V_REGN
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