MAX1647EAP Maxim Integrated Products, MAX1647EAP Datasheet - Page 15

• Manufacturer Part Number: MAX1647EAP
• Description: Battery Charger Lead-Acid/Li-Ion/NiCD/NiMH 4000mA 18V 20-Pin SSOP
• Manufacturer: Maxim Integrated Products
• Type: Battery Chargerr

Specifications of MAX1647EAP

• Package: 20SSOP
• Battery Type: Lead-Acid|Li-Ion|NiCD|NiMH
• Operating Supply Voltage: 7.5 to 28 V
• Output Current: 4000 mA
• Output Voltage: 18(Max) V

internal current sources’ state, and the six most signifi-
cant bits control the switching regulator’s current. The
internal current source supplies 1mA resolution to the
battery to comply with the smart-battery specification.
When the current is set to a number greater than 32,
the internal current source remains at 31mA. This guar-
antees that battery-current setting is monotonic regard-
less of current-sense resistor choice and current-sense
amplifier offset.
The GMI amplifier’s noninverting input is driven by a 4:1
resistive voltage divider, which is driven by the 6-bit
DAC. If an external 4.096V reference is used, this input
is approximately 1.0V at full scale, and the resolution is
16mV. The current-sense amplifier drives the inverting
input to the GMI amplifier. It measures the voltage
across the current-sense resistor (R
between the CS and BATT pins), amplifies it by approx-
imately 5.45, and level shifts it to ground. The full-scale
current is approximately 0.2V / R
is 3.2mV / R
The current-regulation-loop is compensated by adding
a capacitor to the CCI pin. This capacitor sets the cur-
rent-feedback loop’s dominant pole. The GMI amplifier’s
output is clamped to between approximately one-fourth
and three-fourths of the REF voltage. While the current is
in regulation, the CCV voltage is clamped to within
80mV of the CCI voltage. This prevents the battery volt-
age from overshooting when the DAC voltage setting is
updated. The converse is true when the voltage is in
regulation and the current is not at the current DAC set-
ting. Since the linear range of CCI or CCV is about 1.5V
to 3.5V or about 2V, the 80mV clamp results in a rela-
tively negligible overshoot when the loop switches from
voltage to current regulation or vice versa.
The battery voltage or current is controlled by the cur-
rent-mode, pulse-width-modulated (PWM), DC-DC con-
verter controller. This controller drives two external
N-channel MOSFETs, which switch the voltage from the
input source. This switched voltage feeds an inductor,
which filters the switched rectangular wave. The con-
troller sets the pulse width of the switched voltage so that
it supplies the desired voltage or current to the battery.
The heart of the PWM controller is the multi-input com-
parator. This comparator sums three input signals to
determine the pulse width of the switched signal, set-
ting the battery voltage or current. The three signals are
the current-sense amplifier’s output, the GMV or GMI
error amplifier’s output, and a slope-compensation sig-
nal, which ensures that the controller’s internal current-
control loop is stable.
SEN
.
Chemistry-Independent Battery Chargers
______________________________________________________________________________________
SEN
PWM Controller
, and the resolution
SEN
) (which is
The PWM comparator compares the current-sense
amplifier’s output to the higher output voltage of either
the GMV or the GMI amplifier (the error voltage). This
current-mode feedback corrects the duty ratio of the
switched voltage, regulating the peak battery current
and keeping it proportional to the error voltage. Since
the average battery current is nearly the same as the
peak current, the controller acts as a transconductance
amplifier, reducing the effect of the inductor on the out-
put filter LC formed by the output inductor and the bat-
tery’s parasitic capacitance. This makes stabilizing the
circuit easy, since the output filter changes from a com-
plex second-order RLC to a first-order RC. To preserve
the inner current-control loop’s stability, slope compen-
sation is also fed into the comparator. This damps out
perturbations in the pulse width at duty ratios greater
than 50%.
At heavy loads, the PWM controller switches at a fixed
frequency and modulates the duty cycle to control the
battery voltage or current. At light loads, the DC current
through the inductor is not sufficient to prevent the cur-
rent from going negative through the synchronous recti-
fier (Figure 3, M2). The controller monitors the current
through the sense resistor R
the synchronous rectifier turns off to prevent negative
current flow.
The MAX1647 drives external N-channel MOSFETs to
regulate battery voltage or current. Since the high-side
N-channel MOSFET’s gate must be driven to a voltage
higher than the input source voltage, a charge pump is
used to generate such a voltage. The capacitor C7
(Figure 3) charges to approximately 5V through D2
when the synchronous rectifier turns on. Since one side
of C7 is connected to the LX pin (the source of M1), the
high-side driver (DHI) can drive the gate up to the volt-
age at BST, which is greater than the input voltage,
when the high-side MOSFET turns on.
The synchronous rectifier behaves like a diode, but with
a smaller voltage drop to improve efficiency. A small
dead time is added between the time that the high-side
MOSFET turns off and the synchronous rectifier turns
on, and vice versa. This prevents crowbar currents (cur-
rents that flow through both MOSFETS during the brief
time that one is turning on and the other is turning off).
Connect a Schottky rectifier from ground to LX (across
the source and drain of M2) to prevent the synchronous
rectifier’s body diode from conducting. The body diode
typically has slower switching-recovery times, so allow-
ing it to conduct would degrade efficiency.
SEN
; when it drops to zero,
MOSFET Drivers
15