MAX8731 Maxim Integrated Products, MAX8731 Datasheet - Page 23

• Manufacturer Part Number: MAX8731
• Description: SMBus Level 2 Battery Charger
• Manufacturer: Maxim Integrated Products

Clamping the other two control loops close to the low-
est control loop ensures fast transition with minimal
overshoot when switching between different control
loops (see the Compensation section).
With sufficient charge current, the MAX8731’s inductor
current never crosses zero, which is defined as contin-
uous-conduction mode. The regulator switches at
400kHz (nominal) if V
troller starts a new cycle by turning on the high-side
MOSFET and turning off the low-side MOSFET. When
the charge-current feedback signal (CSI) is greater
than the control point (LVC), the CCMP comparator out-
put goes high and the controller initiates the off-time by
turning off the high-side MOSFET and turning on the
low-side MOSFET. The operating frequency is gov-
erned by the off-time and is dependent upon V
V
The on-time can be determined using the following
equation:
where:
The switching frequency can then be calculated:
These equations describe the controller’s pseudo-
fixed-frequency performance over the most common
operating conditions.
At the end of the fixed off-time, the controller initiates a
new cycle if the control point (LVC) is greater than
100mV and the peak charge current is less than the
cycle-by-cycle current limit. Restated another way,
CSSP
. The off-time is set by the following equation:
t
OFF
I
SMBus Level 2 Battery Charger with
t
RIPPLE
Continuous-Conduction Mode
ON
______________________________________________________________________________________
=
f
SW
2 5 . µ
=
CSIN
V
=
CSSN
=
s
L I
t
×
ON
V
×
BATT
V
< 0.88 x V
RIPPLE
CSSP
+
−
1
t
V
V
OFF
L
CSSP
BATT
×
t
−
OFF
V
CSIN
CSSP
. The con-
CSIN
and
IMIN must be high, IMAX must be low, and OVP must
be low for the controller to initiate a new cycle. If the
peak inductor current exceeds the IMAX comparator
threshold or the output voltage exceeds the OVP
threshold, then the on-time is terminated. The cycle-by-
cycle current limit effectively protects against overcur-
rent and short-circuit faults.
If during the off-time the inductor current goes to zero,
the ZCMP comparator output pulls high, turning off the
low-side MOSFET. Both the high- and low-side
MOSFETs are turned off until another cycle is ready to
begin. ZCOMP causes the MAX8731 to enter into dis-
continuous-conduction mode (see the Discontinuous
Conduction section).
There is a 0.3µs minimum off-time when the (V
V
V
time is reached. The switching frequency in this mode
varies according to the equation:
The MAX8731 can also operate in discontinuous-con-
duction mode to ensure that the inductor current is
always positive. The MAX8731 enters discontinuous-
conduction mode when the output of the LVC control
point falls below 100mV. This corresponds to peak
inductor current = 500mA:
charge current for RS2 = 10mΩ.
In discontinuous mode, a new cycle is not started until
the LVC voltage rises above 100mV. 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 charger is in constant-voltage mode
with a nearly full battery pack.
CSIN
CSSP
) differential becomes too small. If V
, then the threshold for the 0.3µs minimum off-
f
I
CHG
Remote Sense
=
V
CSSN
L I
=
Discontinuous Conduction
×
1
2
RIPPLE
×
−
20
V
100
BATT
1
×
mV
RS
+
2
0 3 . µ
=
250
s
mA
CSIN
≥ 0.88 x
CSSP
23
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