BQ24618_15 Datasheet, PDF (29/43 Pages) Texas Instruments – System Power Selector and Low

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bq24618
SLUSA55B – OCTOBER 2010 – REVISED APRIL 2015
I_COUT = ^IRIPPLE » 0.29 ´ I_RIPPLE
2´ 3
(15)
The output capacitor voltage ripple can be calculated as follows:
DV_o
=
1
8LCf_s2
æ
ççè V_BAT
-
V_BAT2
V_IN
ö
÷÷ø
(16)
At certain input/output voltage and switching frequency, the voltage ripple can be reduced by increasing the
output filter LC.
The bq24618 has an internal loop compensator. To get good loop stability, the resonant frequency of the output
inductor and output capacitor should be designed from 12 kHz to 17 kHz. The preferred ceramic capacitor is 25-
V or higher rating, X7R or X5R for 4-cell applications.
9.2.2.4 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. MOSFETs of 30-V or higher voltage rating 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 proper the MOSFET, based on a trade-off 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.
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 ON-resistance r_DS(on)), input voltage (V_IN), switching frequency (f_S),
turnon time (t_on) and turnoff time (t_off):
( ) 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 a 100ºC junction
temperature rise. The second term represents the switching loss. The MOSFET turnon and turnoff times are
given by:
t_on
=
Q_SW
I_on
,
t _off
=
Q_SW
I_off
where
• Q_sw is the switching charge.
• I_on is the turnon gate-drive current.
• I_off is the turnoff gate-drive current.
(19)
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):
1
Q_SW = Q_GD + ₂ ´ Q_GS
(20)
Total gate drive 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:
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:
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