MAX17085BETL+T Maxim Integrated Products, MAX17085BETL+T Datasheet - Page 30

• 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
where k
due to DC voltage bias; k
rated capacitors.
For f
is the closest common capacitor for C
If the internal resistance of the battery is close to the ESR
of the output capacitor, the voltage ripple is shared with
the battery, and is less than calculated.
Firmly establish the input voltage range and maximum
load current before choosing a switching frequency and
inductor operating point (ripple-current ratio). The pri-
mary design trade-off lies in choosing a good switching
frequency and inductor operating point, and the follow-
ing four factors dictate the rest of the design:
U Input Voltage Range: The maximum value (V
U Maximum Load Current: There are two values to
U Switching Frequency: This choice determines the
U Inductor Operating Point: This choice provides
30
must accommodate the worst-case, high AC-adapter
voltage. The minimum value (V
for the lowest battery voltage after drops due to con-
nectors, fuses, and battery selector switches. If there
is a choice at all, lower input voltages result in better
efficiency.
consider. The peak load current (I
mines the instantaneous component stresses and fil-
tering requirements and thus drives output capacitor
selection, inductor saturation rating, and the design
of the current-limit circuit. The continuous load cur-
rent (I
drives the selection of input capacitors, MOSFETs,
and other critical heat-contributing components.
basic trade-off between size and efficiency. The opti-
mal frequency is largely a function of maximum input
voltage due to MOSFET switching losses that are
proportional to frequency and V
quency is also a moving target, due to rapid improve-
ments in MOSFET technology that are making higher
frequencies more practical.
trade-offs between size vs. efficiency and transient
response vs. output ripple. Low inductor values pro-
SW
_____________________________________________________________________________________
C
OUT(CHG)
CAP-BIAS
= 1.2MHz, I
Main SMPS Design Procedure
LOAD
) determines the thermal stresses and thus
is the derating factor for the capacitor
=
RIPPLE
f
SW
× × D
I
RIPPLE
CAP-BIAS
8
= 1A, DV
V
BATT
SYS(MIN)
IN 2
is typically 2 for 25V
BATT
×
LOAD(MAX)
. The optimum fre-
OUT(CHG)
k
CAP-BIAS
= 70mV, 4.7FF
) must account
SYS(MAX)
.
) deter-
)
The switching frequency and inductor operating point
determine the inductor value as follows:
For example: I
5V, f
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite
cores are often the best choice, although powdered iron
is inexpensive and can work well at 200kHz. The core
must be large enough not to saturate at the peak induc-
tor current (I
Most inductor manufacturers provide inductors in stan-
dard values, such as 1.0FH, 1.5FH, 2.2FH, 3.3FH, etc.
Also look for nonstandard values, which can provide
a better compromise in LIR across the input voltage
range. If using a swinging inductor (where the no-load
inductance decreases linearly with increasing current),
evaluate the LIR with properly scaled inductance values.
Output capacitor selection is determined by the control-
ler stability requirements, and the transient soar and sag
requirements of the application.
vide better transient response and smaller physical
size, but also result in lower efficiency and higher
output ripple due to increased ripple currents. The
minimum practical inductor value is one that causes
the circuit to operate at the edge of critical conduc-
tion (where the inductor current just touches zero with
every cycle at maximum load). Inductor values lower
than this grant no further size-reduction benefit. The
optimum operating point is usually found between
20% and 50% ripple current. For high-duty-cycle
applications, select an LIR value of ~ 0.4. When pulse
skipping (SKIP high and light loads), the inductor
value also determines the load-current value at which
PFM/PWM switchover occurs.
SW
= 600kHz, 40% ripple current or LIR = 0.4:
L
=
PEAK
I
12V 600kHz 8A 0.4
LOAD(MAX)
L
PEAK
=
):
V
×
5V
SYS SW LOAD(MAX)
V
=
Output Capacitor Selection
OUT
×
I
LOAD(MAX)
f
(
12V - 5V
(
= 8A, V
V
I
SYS
×
- V
)
Inductor Selection
×
SYS



1
OUT
+
LIR
= 12V, V
LIR
=
2
)
1.5 H



F
OUT
=