BQ24702_16 Datasheet, PDF (25/37 Pages) Texas Instruments – MULTICHEMISTRY BATTERY CHARGER CONTROLLER

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SLUS553E – MAY 2003 – REVISED OCTOBER 2007
PWM SELECTOR SWITCH GATE DRIVE
Because the external P-channel MOSFETs (as well as the internal MOSFETs) have a maximum gate-source
voltage limitation of the input voltage, VCC, cannot be used directly to drive the MOSFET gate under all input
conditions. To provide safe MOSFET-gate-drive at input voltages of less than an intermediate gate drive voltage
rail was established (VSHP). Where V_HSP = VCC – 10 V. This ensures adequate enhancement voltage across all
operating conditions.
An external zener diode (D3) connected between VCC and VHSP is required for transient protection; its
breakdown voltage should be above the maximum value for internal VHSP/VCC clamp voltage for all operating
conditions.
TRANSIENT CONDITIONS AT SYSTEM, OVER-VOLTAGE AT SYSTEM TERMINAL
Overshoot conditions can be observed at the system terminal due to fast load transients and inductive
characteristics of the system terminal to load connection. An overshoot at the system terminal can be directly
coupled to the VCC and VBAT nodes, depending on the switch mode of operation. If the capacitors at VBAT and
VCC can not reduce this overshoot to values below the absolute maximum ratings, it is recommended that an
additional capacitor is added to the system terminal to avoid damage to IC or external components due to
voltage overstress under those transient conditions.
AC ADAPTER COLLAPSING DUE TO TRANSIENT CONDITIONS
The ac adapter voltage collapses when the ac switch is on and a current load transient at the system exceeds
the adapter current limit protection. Under those conditions the ac switch is turned off when the ac adapter
voltage falls below the ac adapter detection threshold. If the system terminal to load impedance has an inductive
characteristic, a negative voltage spike can be generated at the system terminal and coupled into the battery line
via the battery switch backgate diode.
In normal operation, with a battery present, this is not an issue, as the low battery impedance holds the voltage
at battery line. However, if a battery is not present or the pack protector switches are open the negative spike at
the system terminal is directly coupled to the SRP/SRN pins via the R19/R21 resistors.
Avoid damage to the SRP/SRN pins if this transient condition happens in the application. If a negative voltage
spike happens at system terminal and R19/R21 limit the current sourced from the pin to less than –50 mA (Ipin =
Vsystem/R19), the pins SRP/SRN are not damaged and the external protection schottky diodes are not required.
However, if the current under those transient conditions exceeds –50 mA, external schottky diodes must be
added to clamp the voltage at pins SRP/SRN so they do not exceed the absolute maximum ratings specified
(-0.3 V).
IBAT AMPLIFIER
A filter with a cutoff frequency smaller than 10 kHz should be added to the IBAT output to remove switching
noise.
POWER DISSIPATION CALCULATION
During PWM operation, the power dissipated internally to the IC increases as the internal driver is switching the
PWM FET on/off. The power dissipation figures are dependent on the external FET used, and can be calculated
using the following equation:
Pd(max) = [IDDOP + Qg × Fs(max)] × VADAP
where:
Qg = Total gate charge for selected PWM MOSFET
IDDOP = Maximum quiescent current for IC
VADAP = Maximum adapter voltage
Fs(max) = Maximum PWM switching frequency
The maximum junction temperature for the IC must be limited to 125°C, under worst case conditions.
Copyright © 2003–2007, Texas Instruments Incorporated
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