AD8450ASTZ datasheet(37/42 Pages) AD

37 / 42 page

AD8450

Data Sheet

Rev. B | Page 36 of 41

Overload Comparators

The AD8450 features identical fault sensing comparators for

overcurrent (OCPS pin) and overvoltage (OVPS pin) to help

protect against battery damage. The reference pins, OVPR and

OCPR, are hardwired via 0 Ω resistors, R27 and R28, to the 2.5 V

reference. The outputs of the comparators are connected together

internally, and are active low in the event of an overdrive of

either parameter.

For reference, the sense pins are set at 20% greater than the

reference. For other sense voltage ratios, simply calculate a new

value for the resistor divider. The 2.49 kΩ resistor was selected

as an easy equivalent to the 2.5 V reference, the 499 Ω resistors

to the ISMEA and BVMEA as 20% greater. These values were

selected for an experimental 1 A charge/discharge system built

in the lab. Other ratios and values are user selected.

As a basic test or experiment, simply apply enough voltage at

the PGIA or PGDA inputs to exceed 3 V at ISMEA or BVMEA.

The FAULT output pin switches from 5 V to 0 V if either input

exceeds the sense trigger level.

VSET Buffer

The VSET buffer is a unity gain, voltage follower pin accessible

for testing. Apply a voltage up to 5 V at the VSET input, and

measure the output at TPVSETBF.

CV and CC Loop Filter Amplifiers

The constant voltage (CV) and constant current (CC) integrators

are identical circuits and are the two active integrator elements of

the master loop compensation and switching block (see Figure 49).

Except for their external connections, the two circuits are identical

and are tested in the same way, sequentially. The integrator

outputs are analog OR’ed together, creating the VCTRL output to

the input of an external pulse-width modulation controller.

As shown in Figure 49, the integrator op amp inputs are called

IVE0, IVE1, VVE0, VVE1, and VVP0. The first two letters (IV

or VV) signify the constant current or constant voltage integrator.

The third letter identifies the noninverting input (P) or the

inverting input (E for error input). The final digit (0 or 1)

indicates the state of the mode circuit (0 for discharge and 1

for charge). Because the integrators are connected in parallel,

a static test of either integrator requires disabling the other by

forcing the output to the supply rail, reverse biasing the

transistor/diode gate.

CC and CV Integrator Tests

The RUN_TEST1 and RUN_TEST2 switches provide all the circuit

switching required to test the integrator. Set RUN_TEST1 to the

TEST position and apply 2.5 V to the ISET and VSET inputs;

then read 2.5 V at the VCTRL output. Set RUN_TEST2 to

TEST_CC, then TEST_CV, and the VCTRL output voltage still

measures 2.5 V.

Uncommitted Op Amp

The uncommitted op amp is configured as a follower (R24 is

installed between the OAVN pin and the OAVO pin). The input

pin, OAVP, is jumper connected to ground via OAVP. To test

the uncommitted op amp, simply connect a jumper from TP2.5V

and Pin 1 of Jumper OAVP. The output TPOAVO reads 2.5 V.

USING THE AD8450

Except for the power converter and accessories, such as filters

and current sensing, the AD8450-EVALZ includes all of the

signal path elements necessary to implement a battery charging/

forming system (see Figure 59).

The AD8450 is usable with either linear or switch mode power

converters. Switching converters typically generate higher noise

levels than linear; however, switching converters are the most

popular by far because of significantly higher efficiency and

lower cost. Regardless of the power converter architecture used, the

PID loop must be configured to reflect the phase shift and gain of

the power stage. Circuit simulation is helpful with this task.

On the right hand side of Figure 62 are four universal loop

compensation circuits. All or part of the circuits are usable

for installing fixed components when the AD8450-EVALZ is

connected to a battery system for design verification. There are

two feedback amplifiers, but four potentially distinctive separate

configurations. The types and values of passive components

vary according to the power converter and its characteristics.

To use the board for setting up a charging system, replace the

10 kΩ resistors in the feedback (there are no capacitors installed)

and connect the measured feedback voltages by installing

jumpers RUN1 through RUN5. Remove jumpers TST1 through

TST5 and install capacitors associated with the integrator.

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