MAX745CSLASHD datasheet(7/9 Pages) MAXIM

7 / 9 page

To set currents below full scale without changing R1,

adjust the voltage at SETI according to the following

formula:

ICHG = IFS (VSETI / VREF)

A capacitor at CCI sets the current-feedback loop’s

dominant pole. While the current is in regulation, CCV

voltage is clamped to within 80mV of the CCI voltage.

This prevents the battery voltage from overshooting when

the voltage setting is changed. The converse is true when

the voltage is in regulation and the current setting is

changed. Since the linear range of CCI or CCV is about

2V (1.5V to 3.5V), the 80mV clamp results in negligible

overshoot when the loop switches from voltage regulation

to current regulation, or vice versa.

Monitoring Charge Current

The battery-charging current can be externally monitored

by placing a scaling resistor (RIBAT) between IBAT and

GND. IBAT is the output of a voltage-controlled current

source, with output current given by:

IBAT

SENSE

0.9 A

x V

where VSENSE is the voltage across the current-sense

resistor (in millivolts) given by:

VSENSE = VCS - VBATT = ICHG x R1

The voltage across RIBAT is then given by:

VIBAT = 0.9 x 10-3 AV x ICHG x R1 x RIBAT

RIBAT must be chosen to limit VIBAT to voltages below

2V for the maximum charging current. Connect IBAT to

GND if unused.

PWM Controller

The battery voltage or current is controlled by a current-

mode, PWM DC/DC converter controller. This controller

drives two external N-channel MOSFETs, which control

power from the input source. The controller sets the

switched voltages pulse width so that it supplies the

desired voltage or current to the battery. Total component

cost is reduced by using a dual, N-channel MOSFET.

The heart of the PWM controller is a multi-input com-

parator. This comparator sums three input signals to

determine the switched signal’s pulse width, setting the

battery voltage or current. The three signals are the

current-sense amplifier’s output, the GMV or GMI error

amplifier’s output, and a slope-compensation signal that

ensures that the current-control loop is stable.

The PWM comparator compares the current-sense

amplifier’s output to the lower output voltage of either the

GMV or GMI amplifiers (the error voltage). This current-

mode feedback reduces the effect of the inductor on the

output filter LC formed by the output inductor (L1) and C1

(Figure 1). This makes stabilizing the circuit much easier,

since the output filter changes to a first-order RC from a

complex, second-order RLC.

Figure 2. Functional Diagram

MAX745

Switch-Mode Lithium-Ion

Battery-Charger

www.maximintegrated.com

Maxim Integrated │ 7

BATT

1/4

IBAT

DCIN

CURRENT

SENSE

AV = 6

SETI

CCI

VADJ

CCV

CELL0

BST

STATUS

REF

DHI

DLO

PGND

GND

GMV

GMI

CELL1

PWM

LOGIC

5.4V

REG

4.2

REF

CELL

LOGIC

CLAMP

REF

THM/SHDN

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