BQ24735_17 Datasheet, PDF (30/46 Pages) Texas Instruments – 1- to 4-Cell Li+ Battery SMBus Charge Controller for Supporting Turbo Boost Mode With N-Channel Power MOSFET Selector

bq24735
SLUSAK9B – SEPTEMBER 2011 – REVISED APRIL 2015
www.ti.com
Ceramic capacitors show a DC-bias effect. This effect reduces the effective capacitance when a DC-bias voltage
is applied across a ceramic capacitor, as on the output capacitor of a charger. The effect may lead to a
significant capacitance drop, especially for high output voltages and small capacitor packages. See the
manufacturer's data sheet about the performance with a DC-bias voltage applied. It may be necessary to choose
a higher voltage rating or nominal capacitance value in order to get the required value at the operating point.
10.2.2.7 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 6 V of gate drive voltage. A 30-V or higher voltage rating MOSFETs are
preferred for 19- to 20-V 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 the 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 the 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) x Q_GD; FOM_bottom = R_DS(on) x Q_G
(8)
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
(9)
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 turnon and turnoff times are
given by:
t_on
=
Q_{SW ,}
I_on
t_off
=
Q_SW
I_off
(10)
where Q_sw is the switching charge, I_on is the turnon gate driving current and I_off is the turnoff gate driving current.
If the switching charge is not given in 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
(11)
Gate driving current can be estimated by 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_REGN - V_{plt ,}
R_on
I_off
=
V_plt
R_off
(12)
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) x I_CHG ² x R_DS(on)
(13)
When charger operates in nonsynchronous 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), nonsynchronous mode charging current (I_NONSYNC), and duty cycle (D).
P_D = V_F x I_NONSYNC x (1 - D)
(14)
The maximum charging current in nonsynchronous mode can be up to 0.25 A for a 10-mΩ charging current
sensing resistor, or 0.5 A if battery voltage is below 2.5 V. 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 nonsynchronous mode charging current.
30
Submit Documentation Feedback
Product Folder Links: bq24735
Copyright © 2011–2015, Texas Instruments Incorporated