Low-Cost, Multichemistry Battery-
Charger Building Block
30
0
-1
-2
-3
-4
-5
-6
RS1
AC ADAPTER
AC LOAD
2A
1A
V
V
= 16V
= 12.3V
DCIN
BATT
V
V
= 16V
= 8.2V
DCIN
BATT
FREQUENCY
MAX1772
20
10
FREQ = 50kHz
FREQ = 125kHz
FREQ = 250kHz
V
V
= 18V
= 16.4V
DCIN
BATT
MAX172
0
-10
0
10 20 30 40 50 60 70 80
DUTY CYCLE (%)
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
(A)
I
RS1
Figure 3. IINP Accuracy vs. V
/V
Figure 4. IINP Accuracy vs. AC Load Duty Cycle
DCIN BATT
Dropout Operation
MOSFETs and Schottky Diodes
The MAX1772 has 99.99% duty-cycle capability with a
10ms maximum on-time and 1µs off-time. This allows
the charger to achieve dropout performance limited
only by resistive losses in the DC-DC converter compo-
nents (D1, N1, RS1, RS2) (Figure 1). The actual dropout
voltage is limited to 100mV between CSSP and CSIN by
the power-fail comparator.
Schottky diode D1 provides power to the load when the
AC adapter is inserted. This diode must be able to
deliver the maximum current as set by RS1.
The n-channel MOSFETs (N1, N2) are the switching
devices for the buck controller. High-side switch N1
should have a current rating of at least 8A and have an
on-resistance (R
) of 50mΩ or less. The driver for
DS(ON)
N1 is powered by BST; its current should be less than
10mA. Select a MOSFET with a low total gate charge
Compensation
Each of the three regulation loops—the input current
limit, the charging current limit, and charging voltage
limit—can be compensated separately using the CCS,
CCI, and CCV pins, respectively.
(Q
) and determine the required drive current by
GATE
ꢀ
I
= Q
f (where f is the DC-DC converter’s
GATE
GATE
400kHz maximum switching frequency).
The low-side switch (N2) should also have a current rat-
The charge-current-loop error-amp output is brought
out at CCI. Likewise, the source current error-amp out-
put is brought out at CCS; 0.01µF capacitors to ground
at CCI and CCS compensate the current loops in most
charger designs. Raising the value of these capacitors
reduces the bandwidth of these loops.
ing of at least 8A, have an R
of 100mΩ or less,
DS(ON)
and a total gate charge less than 10nC. N2 is used to
provide the starting charge to the BST capacitor (C15).
During normal operation, the current is carried by
Schottky diode D2. Choose a Schottky diode capable
of carrying the maximum charging current.
The voltage-regulating-loop error-amp output is brought
out at CCV. Compensate this loop by connecting a
series RC network from CCV to GND. Recommended
values are 1kΩ and 0.1µF. The zero set by the series
RC increases midfrequency gain to provide phase
compensation. The pole at CCV is set by the capacitor
and the voltage error-amp output impedance at low fre-
quencies to integrate the DC error.
D3 is a signal-level diode, such as the 1N4148. This
diode provides the supply current to the high-side
MOSFET driver.
Inductor Selection
Inductor L1 provides power to the battery while it is
being charged. It must have a saturation current of at
least 4A plus 1/2 of the current ripple (ΔI ):
L
Component Selection
Table 2 lists the recommended components and refers
to the circuit of Figure 1. The following sections describe
how to select these components.
I
= 4A + (1/2) ΔI
(8)
SAT
L
16 ______________________________________________________________________________________
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