MAX1535A Datasheet, PDF (28/39 Pages) Maxim Integrated Products – Highly Integrated Level 2 SMBus Battery Charger

Highly Integrated Level 2 SMBus
Battery Charger
by the off-time and is dependent upon V_DCIN and
V_BATT. The off-time is set by the following equation:
t_OFF
=
2.5µs
×
V_DCIN − V_BATT
V_DCIN
The on-time can be determined using the following
equation:
t_ON
=
L × I_RIPPLE
V_CSSN - V_BATT
w h e r eI_RIPPLE =
V_BATT ×t _{OFF .}
L
The switching frequency can then be calculated:
f_SW
=
t_ON
1
+ t_OFF
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
MAX1535A enters into the discontinuous conduction
mode (see the Discontinuous Conduction section).
There is a 0.3µs minimum off-time when the (V_DCIN -
V_BATT) differential becomes too small. If V_BATT ≥ 0.88 x
V_DCIN, then the threshold for minimum off-time is
reached and the off-time is fixed at 0.27µs. The switch-
ing frequency in this mode varies according to the
equation:
f=
1
L × I_RIPPLE
+ 0.27µs
(V_CSSN - V_BATT)
Discontinuous Conduction
The MAX1535A enters discontinuous-conduction mode
when the output of the LVC control point falls below
100mV. For R2 = 10mΩ, this corresponds to 0.5A:
I_MIN
=
0.1V
20 × 2R2
= 0.5
A
charge
current
for
R2 = 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.
Compensation
The charge voltage, charge current, and input current-
limit regulation loops are compensated separately and
independently at the CCV, CCI, and CCS pins.
CCV Loop Compensation
The simplified schematic in Figure 10 is sufficient to
describe the operation of the MAX1535A when the volt-
age loop (CCV) is in control. The required compensation
network is a pole-zero pair formed with C_CV and R_CV. The
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_ESR is
the equivalent series resistance (ESR) of the charger out-
put capacitor (C_OUT). R_L is the equivalent charger output
load, where R_L = ∆V_BATT / ∆I_CHG. The equivalent output
impedance of the GMV amplifier, R_OGMV, is greater than
10MΩ. The voltage amplifier transconductance, GMV =
0.125µA/mV. The DC-to-DC converter transconductance
is dependent upon the charge current-sense resistor R2:
GM_OUT
=
1
A_CSI ×
R2
where A_CSI = 20, and R2 = 0.01Ω in the typical appli-
cation circuits, so GM_OUT = 5A/V.
The loop transfer function (LTF) is given by:
LTF
=
GM_OUT
×
R_L
×
GMV
×
R_OGMV
×
(1+ SC_OUT
(1+ SC_CV ×
× R_ESR )(1+ SC_CV × R_CV )
R_OGMV )(1+ SC_OUT × R_L)
The poles and zeros of the voltage-loop transfer function
are listed from lowest to highest frequency in Table 11.
Near crossover C_CV is much lower impedance than
R_OGMV. Since C_CV is in parallel with R_OGMV, C_CV domi-
nates the parallel impedance near crossover.
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