BQ24742 Datasheet, PDF (24/35 Pages) Texas Instruments – Li-Ion or Li-Polymer Battery Charger with Low Iq and Accurate Trickle Charge

bq24741, bq24742
SLUS875B – MARCH 2009 – REVISED OCTOBER 2009
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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
(10)
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 ®_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
(11)
The first item represents the conduction loss. Usually MOSFET RDS(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
(12)
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
(13)
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 ®_off) of the gate driver:
I_on =
V_{REG N} -
R_on
V_{plt , Ioff}
=
V_plt
R_off
(14)
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)
(15)
When charger operates in non-synchronous mode, the bottom-side MOSFET is off. As a result all the
freewheeling current goes through the body-diode of the bottom-side MOSFET. The body diode power loss
depends on its forward voltage drop (V_F), non-synchronous mode charging current (I_NONSYNC), and duty cycle (D).
P_D = V_F ´ I_NONSYNC ´ (1 - D)
(16)
The maximum charging current in non-synchronous mode can be up to 2.25A for a 20mΩ charging current
sensing resistor considering IC UCP threshold tolerance. The minimum duty cycle happens at lowest battery
voltage. Choose the bottom-side MOSFET with either an internal Schottky or body diode capable of carrying the
maximum non-synchronous mode charging current.
Input Filter Design
During adapter hot plug-in, the parasitic inductance and input capacitor from the adapter cable form a second
order system. The voltage spike at PVCC pin maybe beyond IC maximum voltage rating and damage IC. The
input filter must be carefully designed and tested to prevent over voltage event on PVCC pin.
There are several methods to damping or limit the over voltage spike during adapter hot plug-in. An electrolytic
capacitor with high ESR as an input capacitor can damp the over voltage spike well below the IC maximum pin
voltage rating. A high current capability TVS Zener diode can also limit the over voltage level to an IC safe level.
However these two solutions may not have low cost or small size.
24
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