MAX1908 datasheet(26/30 Pages) MAXIM

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Low-Cost Multichemistry Battery Chargers

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Choose crossover frequency fCO_CS to be 1/5th the

MAX1908/MAX8724/MAX8765/MAX8765A switching

frequency:

Solving for CCS, CCS = 2nF.

To be conservative, set CCS = 10nF, which sets the

crossover frequency at:

The compensation pole, fP_CS is set at:

Component Selection

Table 3 lists the recommended components and refers

to the circuit of Figure 2. The following sections

describe how to select these components.

Inductor Selection

Inductor L1 provides power to the battery while it is

being charged. It must have a saturation current of at

least the charge current (ICHG), plus 1/2 the current rip-

ple IRIPPLE:

ISAT = ICHG + (1/2) IRIPPLE

Ripple current varies according to the equation:

IRIPPLE = (VBATT)

× tOFF/L

where:

tOFF = 2.5µs

× (VDCIN – VBATT)/VDCIN

VBATT < 0.88

× VDCIN

or:

tOFF = 0.3µs

VBATT > 0.88

× VDCIN

Figure 11 illustrates the variation of ripple current vs.

battery voltage when charging at 3A with a fixed 19V

input voltage.

Higher inductor values decrease the ripple current.

Smaller inductor values require higher saturation cur-

rent capabilities and degrade efficiency. Designs for

ripple current, IRIPPLE = 0.3

× ICHG usually result in a

good balance between inductor size and efficiency.

Input Capacitor

Input capacitor C1 must be able to handle the input

ripple current. At high charging currents, the DC-DC

converter operates in continuous conduction. In this

case, the ripple current of the input capacitor can be

approximated by the following equation:

where:

IC1 = input capacitor ripple current.

D = DC-DC converter duty ratio.

ICHG = battery-charging current.

Input capacitor C1 must be sized to handle the maxi-

mum ripple current that occurs during continuous con-

duction. The maximum input ripple current occurs at

50% duty cycle; thus, the worst-case input ripple cur-

rent is 0.5

× ICHG. If the input-to-output voltage ratio is

such that the DC-DC converter does not operate at a

50% duty cycle, then the worst-case capacitor current

occurs where the duty cycle is nearest 50%.

The input capacitor ESR times the input ripple current

sets the ripple voltage at the input, and should not

exceed 0.5V ripple. Choose the ESR of C1 according to:

The input capacitor size should allow minimal output

voltage sag at the highest switching frequency:

ESR

CHG

=−

PCS

OGMS

0 0016

GMS

kHz

CO CS

210

GMS

kHz

CO CS

RIPPLE CURRENT vs.

BATTERY VOLTAGE

VBATT (V)

0.5

1.0

1.5

15161718

VDCIN = 19V

VCTL = ICTL = LDO

4 CELLS

3 CELLS

Figure 11. Ripple Current vs. Battery Voltage

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