MAX1535 Maxim Integrated Products, MAX1535 Datasheet - Page 28

• Manufacturer Part Number: MAX1535
• Description: Highly Integrated Level 2 Smbus Battery Charger
• Manufacturer: Maxim Integrated Products

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Highly Integrated Level 2 SMBus
Battery Charger
by the off-time and is dependent upon V
V
The on-time can be determined using the following
equation:
The switching frequency can then be calculated:
These equations describe the controller’s pseudofixed-
frequency performance over the most common operat-
ing conditions.
At the end of the fixed off-time, the controller initiates a
new cycle if the control point (LVC) is greater than
100mV (IMIN comparator output is high), and the peak
charge current is less than the cycle-by-cycle limit
(IMAX comparator output is low). If the peak charge cur-
rent exceeds the IMAX comparator threshold, the on-time
is terminated. The IMAX comparator governs the maxi-
mum cycle-by-cycle current limit and is internally set to
10A (when R2 = 10mΩ). The cycle-by-cycle current limit
effectively protects against sudden overcurrent 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. The
MAX1535 enters into the discontinuous conduction
mode (see the Discontinuous Conduction section).
There is a 0.3µs minimum off-time when the (V
V
V
reached and the off-time is fixed at 0.3µs. The switch-
ing frequency in this mode varies according to the
equation:
28
where I
BATT
BATT
DCIN
______________________________________________________________________________________
. The off-time is set by the following equation:
) differential becomes too small. If V
, then the threshold for minimum off-time is
RIPPLE
t
OFF
f
=
=
t
=
ON
(
V
f
V
2 5 . µ
SW
BATT
CSSN
L I
=
×
=
s
V
RIPPLE
L
CSSN
×
×
t
L I
- V
ON
t
V
×
OFF
BATT
DCIN
RIPPLE
1
+
1
-
V
t
V
.
OFF
DCIN
BATT
)
−
+
V
0 3
BATT
.
µ
s
BATT
DCIN
≥ 0.88 x
DCIN
and
-
The MAX1535 enters discontinuous-conduction mode
when the output of the LVC control point falls below
100mV. For R2 = 10mΩ, this corresponds to 0.5A:
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.
The charge voltage, charge current, and input current-
limit regulation loops are compensated separately and
independently at the CCV, CCI, and CCS pins.
The simplified schematic in Figure 10 is sufficient to
describe the operation of the MAX1535 when the voltage
loop (CCV) is in control. The required compensation net-
work is a pole-zero pair formed with C
pole is necessary to roll off the voltage loop’s response at
low frequency. The zero is necessary to compensate the
pole formed by the output capacitor and the load. R
the equivalent series resistance (ESR) of the charger out-
put capacitor (C
load, where R
impedance of the GMV amplifier, R
10MΩ. The voltage amplifier transconductance, GMV =
0.125µA/mV. The DC-to-DC converter transconductance
is dependent upon the charge current-sense resistor R2:
where A
cation circuits, so GM
The loop transfer function (LTF) is given by:
The poles and zeros of the voltage-loop transfer function
are listed from lowest to highest frequency in Table 11.
Near crossover C
R
nates the parallel impedance near crossover.
LTF GM
OGMV
I
MIN
=
=
. Since C
OUT
20 2 2
CSI
0 1
×
×
.
R
= 20, and R2 = 0.01Ω in the typical appli-
V
L
R
L
×
= ∆V
OUT
GMV R
CV
=
GM
0 5
CV
.
). R
×
is in parallel with R
BATT
OUT
OUT
OGMV
A ch
is much lower impedance than
L
is the equivalent charger output
=
/ ∆I
= 3.33A/V.
×
Discontinuous Conduction
A
arg
(
(
1
CCV Loop Compensation
CSI
1
CHG
+
+
SC
e current for R
SC
1
×
CV
. The equivalent output
OUT
R
OGMV
2
×
R
×
Compensation
OGMV
R
OGMV,
CV
ESR
, is greater than
)(
and R
)(
1
1
+
+
SC
SC
C
2 10
CV
CV
=
OUT
CV
×
ESR
domi-
. The
R
×
m
CV
R
Ω
L
)
is
)