MAX17085BETL+ Maxim Integrated Products, MAX17085BETL+ Datasheet - Page 34

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

Specifications of MAX17085BETL+

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

Integrated Charger, Dual Main Step-Down
Controllers, and Dual LDO Regulators
The boost capacitors (C
enough to handle the gate charging requirements of the
high-side MOSFETs. Select the boost capacitors to avoid
discharging the capacitor more than 200mV while charg-
ing the high-side MOSFETs’ gates:
where N is the number of high-side MOSFETs used for
one regulator, and Q
in the MOSFET’s data sheet. For example, assume (1)
FDS6298 n-channel MOSFETs are used on the high side.
According to the manufacturer’s data sheet, a single
FDS6298 has a maximum gate charge of 19nC (V
= 5V). Using the above equation, the required boost
capacitance would be:
Selecting the closest standard value, this example
requires a 0.1FF ceramic capacitor.
The input current limit should be set based on the cur-
rent capability of the AC adapter and the tolerance of
the input current limit. The upper limit of the input current
threshold should never exceed the adapter’s minimum
available output current. For example, if the adapter’s
output current rating is 5A P 10%, the input current limit
should be selected so that its upper limit is less than 5A
x 0.9 = 4.5A. Since the input current-limit accuracy of the
MAX17085B is P 2%, the typical value of the input cur-
rent limit should be set at 4.5A divided by 1.02 ≈ 4.41A.
The lower limit for input current must also be considered.
For chargers at the low end of the specification, the input
current limit for this example could be 4.41A x 0.95 or
approximately 4.19A.
The minimum adapter voltage threshold is used to calcu-
late the resistor values at ACIN:
where V
To minimize power loss, choose a large value for R
and calculate R
34
_____________________________________________________________________________________
V
ADP(MIN)
ACIN-ACOK
Setting Charger Input Current Limit
C
ACIN2
Applications Information
BST
R
C
ACIN1
is 1.5V (typ).
BST
GATE
.
=
R
1 10nC
ACIN2
200mV
×
=
+
BST
R
N Q
is the gate charge specified
ACIN2
AC Adapter Detection
×
200mV
) selected must be large
GATE
=
0.05 F
Boost Capacitors
=
V
F
ACIN-ACOK
ACIN1
GS
,
For example:
then:
The nearest standard resistor value for R
The ACOV threshold is then determined by:
where V
Using the values in the example above, V
The relearn function is easily implemented in the
MAX17085B by configuring the system to override the
PDSL gate drive to the adapter and battery selector
MOSFETs as shown in Figure 1. The system initiates
the relearn cycle by disabling the adapter selector
MOSFET and enabling the battery selector MOSFET. The
MAX17085B relies on the system to monitor the battery
discharge voltage. When the battery reaches its critical
discharge voltage threshold, the system reenables the
adapter selector MOSFET.
Important: Keep ISET low during the relearn cycle.
When the relearn cycle is completed, release PDSL first,
wait 10ms, then enable charging.
The output voltage for continuous-conduction opera-
tion is restricted by the nonadjustable minimum off-time
one-shot. For best dropout performance, use the slower
(200kHz) on-time setting. When working with low input
voltages, the duty-factor limit must be calculated using
worst-case values for on- and off-times. Also, keep
in mind that transient response performance of buck
regulators operated too close to dropout is poor, and
bulk output capacitance must often be added (see the
Design Procedure section).
The absolute point of dropout is when the inductor current
ramps down during the minimum off-time (DI
much as it ramps up during the on-time (DI
h = DI
the inductor current higher in response to increased load,
and must always be greater than 1. As h approaches 1,
the absolute minimum dropout point, the inductor current
cannot increase as much during each switching cycle,
and V
capacitance is used.
R
ACIN2
UP
SAG
ACIN-ACOV
/DI
V
ADP(OV)
DOWN
greatly increases unless additional output
=
Main SMPS Dropout Performance
(
V
ADP(MIN)
indicates the controller’s ability to slew
V
=
R
is 2.1V (typ).
ADP(MIN)
ACIN1
V
ACIN-ACOV
R
V
ACIN1
= 249kI
ACIN - ACOK
= 17V
Relearn Application



1
+
R
R
ADP(OV)
ACIN2
ACIN2
ACIN1
- 1
)
UP
=
24.1k
). The ratio



DOWN
is 24.3kI.
is 23.7V.
Ω
) as