MAX17085BETL+ Maxim Integrated Products, MAX17085BETL+ Datasheet - Page 35

• 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

A reasonable minimum value for h is 1.5 for most normal
regulators. With the extended on-time feature, the mini-
mum h value of 1 can be used. Adjusting this up or down
allows trade-offs between V
and minimum operating voltage. For a given value of h,
the minimum operating voltage can be calculated as:
where V
(see the On-Time One-Shot section) and t
the Electrical Characteristics.
If the calculated V
minimum input voltage, then operating frequency must
be reduced or output capacitance added to obtain an
acceptable V
calculate V
Dropout Design Example:
V
250ns, V
Therefore, V
operation. Input transient sags down to 5.5V during an
output load transient are acceptable due to the extended
on-time feature.
The MAX17085B provides a simple way to generate
and valley regulate an auxiliary charge pump to provide
a low-power, high-voltage (12V to 15V) supply for load
switch gate drive bias. Figure 8 shows the charge-pump
application circuit. The charge pump is driven by the
DL pin to boost the output to the desired bias voltage
(V
where V
Connect a resistor-divider from the high-voltage output
to the SKIP pin as shown in Figure 8. When the voltage
at the SKIP pin drops to 2.1V, which is the typical falling-
edge threshold between SKIP mode and forced-PWM
mode, the MAX17085B enters forced-PWM operation,
recharging the bias output. This automatic refresh opera-
tion allows the MAX17085B to remain in skip mode for
OUT
CHG-PUMP
= 5V, f
CHG
F
CHG
is the forward voltage drop of the diodes.
SAG
IN
is the parasitic voltage drop in the charge path
):
SAG
V
SW
= 100mV, h = 1:
V
Integrated Charger, Dual Main Step-Down
IN(MIN)
V
must be greater than 6V for steady-state
IN(MIN)
to be sure of adequate transient response.
CHG - PUMP
. If operation near dropout is anticipated,
______________________________________________________________________________________
= 600kHz, t
IN(MIN)
=
=
1-
1-
Controllers, and Dual LDO Regulators
5V 0.1V
V






is greater than the required
1 250ns
OUT
h t
≈ ×
1.67 s
×
+
SAG
×
SW
3 (5V - V )
OFF(MIN)
T
+
F
SW
, output capacitance,
= 1.67Fs, t
V
CHG



=
Charge Pump
F
6V
OFF(MIN)



OFF(MIN)
is from
=
best efficiency, yet keep the charge pump output above
a minimum threshold. The minimum charge-pump volt-
age is:
Careful PCB layout is critical to achieving low switching
losses and clean, stable operation. The switching power
stage requires particular attention. If possible, mount all
the power components on the top side of the board, with
their ground terminals flush against one another. Follow
these guidelines for good PCB layout:
U Keep the high-current paths short, especially at the
U Keep the power traces and load connections short
Figure 8. Charge-Pump Application
ground terminals. This practice is essential for stable,
jitter-free operation.
and wide. This practice is essential for high efficiency.
Using thick copper PCBs (2oz vs. 1oz) can enhance
full-load efficiency by 1% or more. Correctly routing
PCB traces is a difficult task that must be approached
in terms of fractions of centimeters, where a single
milliohm of excess trace resistance causes a measur-
able efficiency penalty.
CHARGE PUMP
12V TO 15V
5V OUTPUT
C
0.1FF
OUT5
C4
V
CHG-PUMP(MIN)
0.1FF
C2
100kI
R1
L5
D
D
X1
X2
21kI
PCB Layout Guidelines
R2
=
C
0.1FF
N
N
10nF
10nF
BST5
H5
L5
C1
C3
2.1V
×



1
DH5
BST5
LX5
DL5
OUT5
SKIP
+
R2
R1
MAX17085B



35