BQ40Z60_16 Datasheet, PDF (32/47 Pages) Texas Instruments – Programmable Battery Management Unit

bq40z60
SLUSAW3C – DECEMBER 2014 – REVISED JULY 2015
www.ti.com
DV_o
=
1
8LCf_s2
æ
ççè V_BAT
-
V_BAT2
V_IN
ö
÷÷ø
(5)
At a certain input/output voltage and switching frequency, the voltage ripple can be reduced by increasing the
output filter LC. The bq40z60 has an internal loop compensator. To get good loop stability, the resonant
frequency of the output inductor and output capacitor should be designed between 21 kHz and 27 kHz. The
preferred ceramic capacitor has a 25-V or higher rating, X7R or X5R for a 4-cell application.
10.2.2.4 Power MOSFETs Selection
Two external N-CH 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 preferred 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.
FOM_top = R_DS(on) ´ Q_{G D}
FOM_bottom = R_DS(on) ´ Q_G
(6)
The lower the FOM value, the lower the total power loss. Usually a lower r_DS(on) has a 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), turn-on time (t_on), and turn-off time (t_off):
( ) P_top = D ´ I_CHG2
´ R_DS(on) +
1
2
´ V_IN
´ I_CHG
´
t_on + t_off
´ f_S
(7)
The first item represents the conduction loss. Usually MOSFET r_DS(on) increases by 50% with 100°C 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
(8)
where Q_sw is the switching charge, I_on is the turn-on gate-driving current, and I_off is the turn-off 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):
1
Q_SW = Q_GD + ₂ ´ Q_GS
(9)
Total gate-driving current can be estimated by the REGN voltage (V_REGN), MOSFET plateau voltage (V_plt), total
turn-on gate resistance (R_on), 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
(10)
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)
(11)
If the HSRP–HSRN voltage decreases below 5 mV (the charger is also forced into non-synchronous mode when
the average HSRP–HSRN voltage is lower than 1.7 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
1.6 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 non-synchronous
mode charging current.
32
Submit Documentation Feedback
Product Folder Links: bq40z60
Copyright © 2014–2015, Texas Instruments Incorporated