ISYNSET

SYN

SENSE

250 V

R

=

I

x R

W

(5)

ISYNSET

SRP

SRN

3.3V

1 V

R

SENSE

5kW

20x

SYNCH

R

ISYNSET

UCP

I=1V/R

ISYNSET

I

SYN

HIGH ACCURACY IADAPT USING CURRENT SENSE AMPLIFIER (CSA)

INPUT OVERVOLTAGE PROTECTION (ACOV)

INPUT UNDERVOLTAGE LOCK OUT (UVLO)

bq24705

The ISYNSET comparator, or charge undercurrent comparator, compares the voltage between SRP-SRN, and

Figure 29. ISYNSET Comparator Block

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

RC filter is optional, after the 100-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 and charge is disabled. 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.

The system must have a minimum 4 V PVCC voltage to allow proper operation. This PVCC voltage could come

from either input adapter or battery, using a diode-OR input. When the PVCC voltage is below 4 V the bias

circuits REGN and VREF stay inactive, even with ACDET above 0.6 V.

Copyright © 2007–2009, Texas Instruments Incorporated

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