BQ24735_17 Datasheet, PDF (16/46 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
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Feature Description (continued)
The gate drive voltage on ACFET and RBFET is V_CMSRC + 6 V. If the ACFET/RBFET have been turned on for 20
ms, and the voltage across gate and source is still less than 5.9 V, ACFET and RBFET will be turned off. After
1.3-s delay, it resumes turning on ACFET and RBFET. If such a failure is detected seven times within 90
seconds, ACFET/RBFET will be latched off and an adapter removal and system shut down is required to force
ACDET < 0.6 V to reset the IC. After IC reset from latch off, ACFET/RBFET can be turned on again. After 90
seconds, the failure counter will be reset to zero to prevent latch off. With ACFET/RBFET off, charge is disabled.
To turn off ACFET/RBFET, one of the following conditions must be valid:
• In LEARN mode and V_SRN is above battery depletion threshold
• ACOK low
To limit the inrush current on ACDRV pin, CMSRC pin and BATDRV pin, a 4-kΩ resistor is recommended on
each of the three pins.
To limit the adapter inrush current when ACFET is turned on to power system from adapter, the Cgs and Cgd
external capacitor of ACFET must be carefully selected. The larger the Cgs and Cgd capacitance, the slower
turnon of ACFET will be and less inrush current of adapter. However, if Cgs or Cgd is too large, the ACDRV-
CMSRC voltage may still go low after the 20-ms turnon time window is expired. To make sure ACFET will not be
turned on when adapter is hot plugged in, the Cgs value should be 20 times or higher than Cgd. The most cost
effective way to reduce adapter inrush current is to minimize system total capacitance.
9.3.4 Automatic Internal Soft-Start Charger Current
Every time the charge is enabled, the charger automatically applies soft start on charge current to avoid any
overshoot or stress on the output capacitors or the power converter. The charge current starts at 128 mA, and
the step size is 64 mA in CCM mode for a 10-mΩ current sensing resistor. Each step lasts around 240 µs in
CCM mode until it reaches the programmed charge current limit. No external components are needed for this
function. During DCM mode, the soft start up current step size is larger and each step lasts for longer time period
due to the intrinsic slow response of DCM mode.
9.3.5 Converter Operation
The synchronous buck PWM converter uses a fixed-frequency voltage mode control scheme and internal type III
compensation network. The LC output filter gives a characteristic resonant frequency:
1
¦_o =
2p L_oC_o
(1)
The resonant frequency (f_o) is used to determine the compensation to ensure there is sufficient phase margin
and gain margin for the target bandwidth. The LC output filter should be selected to give a resonant frequency of
10–20 kHz nominal for the best performance. Suggested component value as charge current of 750-kHz default
switching frequency is shown in Table 1.
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.
Table 1. Suggested Component Value as Charge Current of Default 750-kHz
Switching Frequency
Charge Current
2A
3A
4A
6A
8A
Output Inductor Lo (µH)
6.8 or 8.2 5.6 or 6.8 3.3 or 4.7
3.3
2.2
Output Capacitor Co (µF)
20
20
20
30
40
Sense Resistor (mΩ)
10
10
10
10
10
16
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