MAX17085BETL+T Maxim Integrated Products, MAX17085BETL+T Datasheet - Page 27

• 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

where C
High-frequency (~ 600kHz nominal) operation optimizes
the application for the smallest component size. Efficiency
trade-off due to higher switching losses is not so signifi-
cant for higher output voltage rails like 5V and 3.3V.
For continuous conduction operation, the actual switch-
ing frequency can be estimated by:
where V
the inductor discharge path, including synchronous rec-
tifier, inductor, and PCB resistances; V
the resistances in the charging path, including the high-
side switch, inductor, and PCB resistances; and t
the on-time calculated by the MAX17085B.
During heavy load transients, the main SMPS can issue
an extended on-time to increase the inductor current
ramp and reduce output voltage sag, thereby reducing
output capacitance requirement. The extended on-time
feature is ideal for high-duty-cycle conditions where the
voltage across the inductor (V
the output voltage. The extended on-time is twice as long
as the normal on-time. A minimum off-time follows after
each extended on-time.
The extended on-time is allowed when the following con-
ditions are met:
U Inductor valley current at the start of the first on pulse
U Greater than 50% duty cycle.
The low-noise forced-PWM mode (SKIP = 1.8V) dis-
ables the zero-crossing comparator, which controls the
low-side switch on-time. This forces the low-side gate-
drive waveform to constantly be the complement of the
high-side gate-drive waveform, so the inductor current
reverses at light loads while DH maintains a duty factor
of V
keep the switching frequency fairly constant. However,
forced-PWM operation comes at a cost: the no-load 5V
bias current remains between 15mA to 35mA per phase
at 600kHz, depending on the MOSFET selection.
is less than 50% of the current-limit setting.
OUT
t
t
TON
DIS
/V
ON5
ON3
SYS
t
SW(NOM)
is the sum of the parasitic voltage drops in
= 6pF.
= V
= V
. The benefit of forced-PWM mode is to
Integrated Charger, Dual Main Step-Down
f
SW
OUT5
OUT3
______________________________________________________________________________________
=
= C
Forced-PWM Mode (SKIP = 1.8V)
t
/V
/V
ON
SYSTEM
SYSTEM
TON
V
Controllers, and Dual LDO Regulators
(V
OUT
SYS
x R
+
SYS
Modes of Operation
+
TON
Extended On-Time
x t
x t
V
V
DIS
SW(NOM)
SW(NOM)
CHG
- V
+ 6.5kI
CHG
OUT
)
) is less than
is the sum of
/1.1
/0.9
ON
is
In skip mode (SKIP = 3.3V or 5V), an inherent automatic
switchover to PFM takes place at light loads. This swi-
tchover is affected by a comparator that truncates the
low-side switch on-time at the inductor current’s zero
crossing sensed across LX and AGND. In discontinuous
conduction (SKIP = 3.3V or 5V, and I
the output voltage has a DC regulation level higher than
the error comparator threshold.
Forcing SKIP low (SKIP = GND) activates a unique
pulse-skipping mode with a minimum switching frequen-
cy of 20kHz. This ultrasonic pulse-skipping mode elimi-
nates audio-frequency modulation that would otherwise
be present when a lightly loaded controller automatically
skips pulses. In ultrasonic mode, the controller automati-
cally transitions to fixed-frequency PWM operation when
the load reaches the same critical conduction point
(I
An ultrasonic pulse occurs (Figure 5) when the control-
ler detects that no switching has occurred within the
last 45Fs. Once triggered, the ultrasonic circuitry pulls
DL high, turning on the low-side MOSFET. This induces
a negative inductor current. A negative current limit
of 72mV protects against excessive negative currents
when DL is turned on.
After the output drops below the regulation voltage, the
controller turns off the low-side MOSFET (DL pulled low)
and triggers a constant on-time (DH driven high). When
the on-time has expired, the controller reenables the low-
side MOSFET until the inductor current drops below the
zero-crossing threshold. Starting with a DL pulse greatly
reduces the peak output voltage when compared to
starting with a DH pulse.
Figure 5. Ultrasonic Waveforms
LOAD(SKIP)
0
) that occurs when normally pulse skipping.
45Fs (typ)
Automatic Pulse-Skipping Mode
Ultrasonic Mode (SKIP = GND)
ZERO-CROSSING
DETECTION
ON-TIME (t
(SKIP = 3.3V or 5V)
OUT
ON
)
INDUCTOR
CURRENT
< I
LOAD(SKIP)
27
),