BQ24740_14 Datasheet, PDF (20/33 Pages) Texas Instruments – Host-Controlled Multi-Chemistry Battery Charger

bq24740
SLUS736C – DECEMBER 2006 – REVISED MARCH 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 very 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,
and 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 cap. 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 ＝ 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
L
´
1
f_s
V_in ´ (1- D) ´ D ´
=
L
1
f_s
(5)
where
V_IN: adapter voltage
V_BAT: BAT voltage
f_S: switching frequency
L: output inductor
D: duty cycle
I_{RIPPLE_MAX} occurs when the duty cycle, D is mostly near to 0.5 at given Vin, fs and L. The ISYNSET comparator,
or charge undercurrent comparator, compares the voltage between SRP-SNR and the threshold set by an
external resistor R_ISYNSET, which can be calculated by:
20
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