BQ24735_17 Datasheet, PDF (28/46 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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Typical Application (continued)
10.2.1 Design Requirements
For this design example, use the parameters listed in Table 7 as the input parameters.
Table 7. Design Parameters
DESIGN PARAMETER
Input Voltage⁽¹⁾
Input Current Limit ⁽¹⁾
Battery Charge Voltage⁽²⁾
Battery Charge Current⁽²⁾
Battery Discharge Current⁽²⁾
EXAMPLE VALUE
17.7 V < Adapter Voltage < 24 V
3.2 A for 65-W adapter
12592 mV for 3-s battery
4096 mA for 3-s battery
6144 mA for 3-s battery
(1) Refer to adapter specification for settings for input voltage and input current limit.
(2) Refer to battery specification for settings.
10.2.2 Detailed Design Procedure
10.2.2.1 Negative Output Voltage Protection
If the battery pack is inserted in reverse order into the charger, output during production or hard shorts on
battery-to-ground generates negative output voltage on the SRP and SRN pins. IC internal electrostatic-
discharge (ESD) diodes from the GND pin to the SRP or SRN pins and two anti-parallel (AP) diodes between the
SRP and SRN pins can be forward-biased and negative current can pass through the ESD diodes and AP
diodes when output has negative voltage. Insert two small resistors for SRP and SRN pins to limit the negative
current level when output has negative voltage. Suggested resistor value is 10 Ω for the SRP pin and 7 to 8 Ω for
the SRN pin. After adding small resistors, the suggested precharge current is at least 192 mA for a 10-mΩ
current-sensing resistor. Another method is using a small diode parallel with output capacitor; when battery
connection is reversed, the diode turns on and limits the negative voltage level. Using diode protection method
without insertion of small resistors into the SRP and SRN pins can get the best charging current accuracy.
10.2.2.2 Reverse Input Voltage Protection
Q6, R12 and R13 in Figure 16 gives system and IC protection from reversed adapter voltage. In normal
operation, Q6 is turned off by negative Vgs. When adapter voltage is reversed, Q6 Vgs is positive. As a result,
Q6 turns on to short gate and source of Q2 so that Q2 is off. Q2 body diode blocks negative voltage to system.
However, CMSRC and ACDRV pins need R10 and R11 to limit the current due to the ESD diode of these pins
when turned on. Q6 must has low Vgs threshold voltage and low Qgs gate charge, so it turns on before Q2 turns
on. R10 and R11 must have enough power rating for the power dissipation when the ESD diode is on. In
Figure 21, the Schottky diode D3 gives the reverse adapter voltage protection, no extra small MOSFET and
resistors are needed.
In Figure 22, the Schottky diode Din is used for the reverse adapter voltage protection.
10.2.2.3 Reduce Battery Quiescent Current
When the adapter is not present, if VCC is powered with voltage higher than UVLO directly or indirectly (such as
through a LDO or switching converter) from battery, the internal BATFET charge pump gives the BATFET pin 6
V higher voltage than the SRN pin to drive the N-channel BATFET. As a result, the battery has higher quiescent
current. This is only necessary when the battery powers the system due to a high system current that goes
through the MOSFET channel instead of the body diode to reduce conduction loss and extend the battery
working life. When the system is totally shut down, it is not necessary to let the internal BATFET charge pump
work. The host controller can use a digital signal EN to disconnect the battery power path to the VCC pin by U2
in Figure 16. As a result, battery quiescent current can be minimized. The host controller still can get power from
BATFET body diode because the total system current is the lowest when the system is shut down, so there is no
high conduction loss of the body diode.
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