MAX17085BETL+T Maxim Integrated Products, MAX17085BETL+T Datasheet - Page 26

• Manufacturer Part Number: MAX17085BETL+T
• Description: Battery Management Dual Main Step-Down Controller
• Manufacturer: Maxim Integrated Products

Specifications of MAX17085BETL+T

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Integrated Charger, Dual Main Step-Down
Controllers, and Dual LDO Regulators
With sufficiently large charge current, the MAX17085B's
inductor current never crosses zero, which is defined
as continuous conduction mode. The controller starts a
new cycle by turning on the high-side MOSFET and turn-
ing off the low-side MOSFET. When the charge-current
feedback signal (CSI) is greater than the control point
(LVC), the CCMP comparator output goes high and the
controller initiates the off-time by turning off the high-
side MOSFET and turning on the low-side MOSFET. The
operating frequency is governed by the off-time and is
dependent upon V
The on-time can be determined using the following equation:
where:
The switching frequency can then be calculated:
At the end of the computed off-time, the controller initi-
ates a new cycle if the control point (LVC) is greater than
5mV (referred at V
current is less than the cycle-by-cycle current limit.
Restated another way, IMIN must be high, IMAX must
be low, and OVP must be low for the controller to initiate
a new cycle. If the peak inductor current exceeds IMAX
comparator threshold or the output voltage exceeds
the OVP threshold, then the on-time is terminated. The
cycle-by-cycle current limit effectively protects against
overcurrent and short-circuit 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. ZCMP
causes the MAX17085B to enter into the discontinuous
conduction mode (see the Discontinuous Conduction
section).
The MAX17085B can also operate in discontinuous
conduction mode to ensure that the inductor current is
always positive. The MAX17085B enters discontinuous
conduction mode when the output of the LVC control
26
_____________________________________________________________________________________
I
RIPPLE
t
ON
CSIN
CSIP
f
SW
=
=
and V
V
=
Continuous Conduction Mode
- V
DCIN
t
L I
V
ON
×
BATT
CSIN
Discontinuous Conduction
RIPPLE
DCIN
+
1
- V
t
), and the peak charge
L
OFF
×
BATT
.
t
OFF
point falls below 5mV (referred at V
RS2 = 20mI, this corresponds to peak inductor current
to be 250mA.
In discontinuous mode, a new cycle is not started until
the LVC voltage rises above 5mV. Discontinuous mode
operation can occur during a conditioning charge of
overdischarged battery packs, when the charge current
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. The charge-current limit loop, CCI, is
compensated internally, while the input current limit and
charge-voltage loops, CCS and CCV, are compensated
externally using a shared capacitor at the CC pin. For
most applications, it is sufficient to place a 10nF capaci-
tor from CC to GND.
The main SMPS of the MAX17085B consists of two inde-
pendent switching regulators that generate a 3.3V and a 5V
output. The regulators use the Quick-PWM control architec-
ture for simplicity, low pin count, and fast transient response.
An extended on-time feature further improves output voltage
sag for high-duty-cycle applications.
The Quick-PWM control architecture is a pseudo-fixed-
frequency, constant on-time, current-mode regulator with
voltage feed-forward. This architecture relies on the output
filter capacitor’s ESR to act as a current-sense resistor, so
the feedback ripple voltage provides the PWM ramp signal.
The control algorithm is simple: the high-side switch on-time
is determined solely by a one-shot whose pulse width is
inversely proportional to input voltage and directly propor-
tional to output voltage. Another one-shot sets a minimum
off-time (270ns typ). The on-time one-shot is triggered if the
error comparator is low, the low-side switch current is below
the valley current-limit threshold, and the minimum off-time
one-shot has timed out.
The heart of the PWM core is the one-shot that sets the
high-side switch on-time. This fast, low-jitter, adjustable
one-shot includes circuitry that varies the on-time in
response to battery and output voltage. The high-side
switch on-time is inversely proportional to the battery
voltage as sensed by the TON input, and proportional to
the output voltage:
Main SMPS Detailed Description
Free-Running Constant-On-Time PWM
Controller with Input Feed-Forward
On-Time One-Shot
CSIP
Compensation
- V
CSIN
). For