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ISYNSET CONTROL (CHARGE UNDER-CURRENT)

I

RIPPLE_MAX

2

and

I

V

V

BAT_MIN

V

IN_MAX

1

fs

L

MIN

(4)

HIGH ACCURACY IADAPT USING CURRENT SENSE AMPLIFIER (CSA)

INPUT OVER VOLTAGE PROTECTION (ACOV)

SLUS736 – DECEMBER 2006

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 very 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.

Whenever the converter goes into 0% duty-cycle mode, and 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 (no 80-ns

recharge pulse), and there is no discharge from the battery.

In bq24740, ISYN is internally set as the charge current threshold at which the charger changes from

non-synchronous operation into 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 can lead to an

over-voltage 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 ½ the inductor current ripple to the full ripple current, where

the inductor current ripple is given by

where

The ISYNSET comparator, or charge under-current comparator, compares the voltage between SRP-BAT and

internal threshold on the cycle-to-cycle base. The threshold is set to 13 mV on the falling edge with 8 mV

hysteresis on the rising edge with 10% variation.

An industry standard, high accuracy current sense amplifier (CSA) is used to monitor the input current by the

host or some discrete logic through the analog voltage output of the IADAPT pin. The CSA amplifies the input

sensed voltage of ACP – ACN by 20x through the IADAPT pin. The IADAPT output is a voltage source 20 times

the input differential voltage. Once PVCC is above 5 V and ACDET is above 0.6V, IADAPT no longer stays at

ground, but becomes active. If the user wants to lower the voltage, they could use a resistor divider from IOUT

to AGND, and still achieve accuracy over temperature as the resistors can be matched their thermal coefficients.

A 200-pF capacitor connected on the output is recommended for decoupling high-frequency noise. An additional

RC filter is optional, after the 200-pF capacitor, if additional filtering is desired. Note that adding filtering also

adds additional response delay.

ACOV provides protection to prevent system damage due to high input voltage. The controller enters ACOV

when ACDET > 3.1 V. Charge is disabled, the adapter is disconnected from the system by turning off ACDRV,

and the battery is connected to the system by turning on BATDRV. ACOV is not latched—normal operation

resumes when the ACDET voltage returns below 3.1 V. ACOV threshold is 130% of the adapter-detect

threshold.

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