ISYNSET CONTROL (SYN AND NON-SYN MODE SETTING)

2

and

1

1

f

f

RIPPLE_MAX

SYN

RIPPLE_MAX

BAT

IN

BAT

IN

IN

S

S

RIPPLE

I

I

I

V

(V

V

)

V

(1 D) D

V

I

L

L

£

£

-

´

´

´ -

´ ´

=

=

(4)

bq24705

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).

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:

where:

L = output inductor

D = duty-cycle

18

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