Acutechnology Semiconductor Inc.

AQ120

www.acutechnology.com

5

Rev. 1.8 April 1, 2006

Operation

The AQ120 is a linear battery charger controller. Charging begins when Vcc rises above

2.5V. When charging, the collector of the external PNP transistor provides the charge current.

Below the target charge voltage the controller is in current mode. When the battery voltage

reaches the target voltage then the controller switches to the voltage mode.

The target voltage is programmable through an external resistor divider. This allows flexibility

to set the voltage (to one or more cells Li ion batteries, NiMH or NiCd batteries).

In current mode the AQ120 regulates the voltage across the sense resistor to be 200 mV.

The charge current is therefore given by Ich = 0.2V / Rsense. An additional resistor is

recommended (as in the typical application diagram) for stability reasons, then the threshold

is actually increased by the value of the resistor multiplied by the current out of the IPROG

pin. The AQ120 is designed for a maximum current in excess of 1A. The external PNP

transistor must have adequate beta, low saturation voltage and sufficient power dissipation

capability. With low supply voltages, the PNP saturation voltage becomes important as well.

In constant voltage mode, the controller will control the battery voltage not to exceed the

target charge voltage.

When Vcc is applied, the charger can be manually shut down by opening the resistor R2 of

the resistor divider (floating the otherwise grounded end of R2) or by pulling high the REF pin.

This can be used in conjunction with an external thermostat switch.

When input power is removed or manual shutdown is entered, the charger will drain only very

small leakage currents from the battery, thus maximizing battery standby time. The leakage

current is due to the reverse-biased base-emitter junction of the external PNP transistor.

A LED indicator can be added to show when the charger is operational, simply connecting

the base-emitter of an additional PNP transistor in parallel to the base-emitter of Q1 and its

collector to the LED.

The AQ120 contains two control loops. To maintain good AC stability in the constant voltage

mode, a capacitor of at least 10uF in series to a 3.3 ohms resistor is usually required from the

collector of the external PNP to ground. The stability of the system is also depending on the

type of external PNP transistor. High beta PNPs may be reducing the phase margin in some

cases. R6 and C3 insure system stability in current mode without affecting the voltage mode.

The use of the resistor R6 affects the threshold. For example for R6=150 ohms the threshold

moves from 200 mV to 215 mV.

Furthermore the use of an external PMOS in place of the PNP represents a valid alternative

to the application when wanting to minimize the current consumption during charge.