MAX1667 Datasheet, PDF (14/28 Pages) Maxim Integrated Products – Chemistry-Independent, Level 2 Smart Battery Charger

Chemistry-Independent,
Level 2 Smart Battery Charger
across the current-sense resistor (R_SEN) (which is
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 0.16V/R_SEN, and the resolution
is 5mV/R_SEN.
The current-regulation loop is compensated by adding a
capacitor to the CCI pin. This capacitor sets the current-
feedback loop’s dominant pole. The GMI amplifier’s out-
put 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 (about 2V), the 80mV clamp results in a relatively
negligible overshoot when the loop switches from volt-
age to current regulation or vice versa.
PWM Controller
The battery voltage or current is controlled by the cur-
rent-mode, PWM, DC-DC converter controller. This con-
troller drives two external N-channel MOSFETs, which
switch the voltage from the input source. This switched
voltage feeds an inductor, which filters the switched rec-
tangular wave. The controller 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.
The PWM comparator compares the current-sense
amplifier’s output to the lower 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 7, M2). The controller monitors the current
through the sense resistor R_SEN; when it drops to zero,
the synchronous rectifier turns off to prevent negative
current flow.
MOSFET Drivers
The MAX1667 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 7) 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 may not be completely
replaced by a diode because the BST capacitor
charges while the synchronous rectifier is turned on.
Without the synchronous rectifier, the BST capacitor
may not fully charge, leaving the high-side MOSFET
with insufficient gate drive to turn on. Use a small MOS-
FET, such as a 2N7002, to guarantee that the BST
capacitor is allowed to charge. In this case, most of the
current at high currents is carried by the Schottky diode
and not by the synchronous rectifier.
Internal Regulator and Reference
The MAX1667 uses an internal low-dropout linear regula-
tor to create a 5.4V power supply (VL), which powers its
internal circuitry. VL can supply up to 20mA, less than
10mA powers the internal circuitry, and the remaining
current can power the external circuitry. The current
used to drive the MOSFETs comes from this supply,
which must be considered when calculating how much
power can be drawn. To estimate the current required to
drive the MOSFETs, multiply the total gate charge of
each MOSFET by the switching frequency (typically
250kHz). To ensure VL stability, bypass the VL pin with a
1µF or greater capacitor.
The MAX1667 has an internal, accurate 4.096V refer-
ence voltage. This guarantees a voltage-setting accu-
racy of ±1% max. Bypass the reference with a 1µF or
greater capacitor.
14 ______________________________________________________________________________________