BQ24620 Datasheet, PDF (25/30 Pages) Texas Instruments – Stand-Alone Synchronous Switch-Mode Lithium Phosphate Battery Charger with Low Iq

bq24620
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SLUS893 – MARCH 2010
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
(15)
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), 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
(16)
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
(17)
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
(18)
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
(19)
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)
(20)
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 shut down.
P_{ICLoss_driver} = V_IN × Q_{g_total} × f_s
(21)
Where Q_{g_total} is the total gate charge for both upper and lower MOSFET at 6V V_REGN.
The VREF load current is another component on VCC input current (Do not overload VREF) where total IC loss
can be described by following equations:
P_VREF = (V_IN - V_VREF ) × I_VREF
P_ICLOSS = P_{ICLOSS _ driver} + P_VREF + P_Quiescent
(22)
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