Low-Cost Multichemistry Battery Chargers

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21

In normal operation, the controller starts a new cycle by

turning on the high-side n-channel MOSFET and

turning off the low-side n-channel MOSFET. When the

charge current is greater than the control point (LVC),

CCMP goes high and the off-time is started. The

off-time turns off the high-side n-channel MOSFET and

turns on the low-side n-channel MOSFET. The opera-

tional frequency is governed by the off-time and is

by the following equations:

where:

These equations result in fixed-frequency operation

over the most common operating conditions.

At the end of the fixed off-time, another cycle begins if

the control point (LVC) is greater than 0.15V, IMIN =

high, and the peak charge current is less than 6A (RS2

= 0.015

Ω), IMAX = high. If the charge current exceeds

IMAX, the on-time is terminated by the IMAX compara-

tor. IMAX governs the maximum cycle-by-cycle current

limit and is internally set to 6A (RS2 = 0.015

Ω). IMAX

protects against sudden overcurrent faults.

If, during the off-time, the inductor current goes to zero,

ZCMP = high, both the high- and low-side MOSFETs

are turned off until another cycle is ready to begin.

≥ 0.88 ×

time of 5ms allows the controller to achieve > 99% duty

cycle in continuous-conduction mode. The switching

frequency in this mode varies according to the equation:

Discontinuous Conduction

The MAX1908/MAX8724/MAX8765 enter discontinuous-

conduction mode when the output of the LVC control

point falls below 0.15V. For RS2 = 0.015

Ω, this corre-

sponds to 0.5A:

for RS2 = 0.015

Ω.

In discontinuous mode, a new cycle is not started until

the LVC voltage rises above 0.15V. Discontinuous-

mode operation can occur during conditioning charge

of overdischarged battery packs, when the charge cur-

rent has been reduced sufficiently by the CCS control

loop, or when the battery pack is near full charge (con-

stant-voltage-charging mode).

MOSFET Drivers

The low-side driver output DLO switches between

PGND and DLOV. DLOV is usually connected through

a filter to LDO. The high-side driver output DHI is boot-

When the low-side driver turns on, BST rises to one

diode voltage below DLOV.

Filter DLOV with a lowpass filter whose cutoff frequency

is approximately 5kHz (Figure 1):

Dropout Operation

The MAX1908/MAX8724/MAX8765 have 99% duty-cycle

capability with a 5ms (max) on-time and 0.3µs (min) off-

time. This allows the charger to achieve dropout perfor-

mance limited only by resistive losses in the DC-DC

converter components (D1, N1, RS1, and RS2, Figure 1).

Replacing diode D1 with a p-channel MOSFET driven by

ACOK improves dropout performance (Figure 2). The

dropout voltage is set by the difference between DCIN

and CSIN. When the dropout voltage falls below 100mV,

the charger is disabled; 200mV hysteresis ensures that

the charger does not turn back on until the dropout volt-

age rises to 300mV.

Compensation

Each of the three regulation loops—input current limit,

charging current limit, and charging voltage limit—are

compensated separately using CCS, CCI, and CCV,

respectively.

f

RC

F

kHz

××

=

1

2

1

233

1

48

ππ

μ

Ω

.

IMIN

V

RS

A

=

×

=

015

20

2

05

.

.

f

LI

VV

s

RIPPLE

CSSN

BATT

=

×

−

()

+

1

03

.

μ

f

tt

ON

OFF

=

+

1

I

Vt

L

RIPPLE

BATT

OFF

=

×

t

LI

VV

ON

RIPPLE

CSSN

BATT

=

×

−

ts

VV

V

OFF

DCIN

BATT

DCIN

=×

−

25

.

μ