BQ24704 Datasheet, PDF (18/31 Pages) Texas Instruments – Low Cost, Host-Controlled Li-Ion and Li-Polymer Battery Charger With Low Iq

bq24704
SLUS838 – MAY 2009........................................................................................................................................................................................................ www.ti.com
During the DCM mode the loop response automatically changes and has a single pole system at which the pole
is proportional to the load current, because the converter does not sink current, and only the load provides a
current sink. This means at low currents the loop response is slower, as there is less sinking current available to
discharge the output voltage. At low currents during non-synchronous operation, there may be a small amount of
negative inductor current during the 80 ns recharge pulse. The charge should be low enough to be absorbed by
the input capacitance.
When BTST – PH < 4 V, the 80-ns recharge pulse occurs on LODRV, the high-side MOSFET does not turn on.
The low-side MOSFET does not turn on (only 80-ns recharge pulse).
ISYNSET CONTROL (SYN AND NON-SYN MODE SETTING)
The ISYNSET pin is used to program the charge current threshold at which the charger changes from
synchronous operation into non-synchronous operation. The low side driver turns on for only 80 ns to charge the
boost capacitor. This is important to prevent negative inductor current, which may cause a boost effect in which
the input voltage increases as power is transferred from the battery to the input capacitors. This boost effect can
lead to an overvoltage on the PVCC node, and potentially cause some damage to the system. This
programmable value allows setting the current threshold for any inductor current ripple, and avoiding negative
inductor current. The minimum synchronous threshold should be set from 1/2 of the inductor current ripple to the
full ripple current, where the inductor current ripple is given by:
I_{RIPPLE_MAX}
2
£ I_SYN £ I_{RIPPLE_MAX}
and
I_RIPPLE =
(V_IN
-
V_BAT
)´
V_BAT
V_IN
´
1
f_S
=
1
V_IN ´(1-D)´D´ _fS
L
L
(4)
where:
V_IN = adapter voltage
V_BAT = BAT voltage
f_{S =} switching frequency
L = output inductor
D = duty-cycle
18
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