Datasheet

MAX8655
Highly Integrated, 25A, Wide-Input,
Internal MOSFET, Step-Down Regulator
16 ______________________________________________________________________________________
Setting the Current Limit
Valley Current Limit
The MAX8655 has an adjustable valley current limit, con-
figurable for foldback with automatic recovery, or con-
stant-current limit with latch-up. To set the constant-
current limit for the latch-up mode, connect a single
resistor R
ILIM2
from ILIM2 to GND. For latch-up current-
limit mode, set R
ILIM2
equal to R
VALLEY
obtained from
the R
VALLEY
vs. Valley Current Limit graph in the Typical
Operating Characteristics section for the required valley
current I
VALLEY
. I
VALLEY
is the value of the inductor val-
ley current at maximum load (I
LOAD(MAX)
-
1
/
2
I
P-P
)
To set the current limit for foldback mode, connect a
resistor from ILIM2 to the output (R
FOBK
), and another
resistor from ILIM2 to GND (R
ILIM2
). See Figure 7. The
values of R
FOBK
and R
ILIM2
are calculated as follows.
First, select the percentage of foldback (P
FB
). This per-
centage corresponds to the current limit when V
OUT
equals zero divided by the current limit when V
OUT
equals its nominal voltage. A typical value of P
FB
is in
the 15% to 40% range. A lower value of P
FB
yields
lower short-circuit current. The following equations are
used to calculate R
FOBK
and R
ILIM2
:
where I
ILIM2
is 5µA.
If the resulting value of R
ILIM2
is negative, increase P
FB
.
Peak Current Limit
The peak current-limit threshold (V
TH
) is set by a resistor
connected from ILIM1 to GND (R
ILIM1
). V
TH
corresponds
to the peak voltage across the sensing element (inductor
or current-sense resistor). R
ILIM1
is calculated as follows:
This allows a maximum DC output current of:
where R
L
is the DC resistance of the inductor.
To ensure maximum output current, use the minimum
value of V
TH
from each setting, and the maximum R
L
values at the highest expected operating temperature.
The DC resistance of the inductor’s copper wire has a
+0.38%/°C temperature coefficient.
An RC circuit is connected across the inductor (see
Figure 8). The RC time constant is set to be 1.1 to 1.2
times the inductor (L/R
L
) time constant. Pick the value
of C9 in the 0.1µF to 0.47µF range, and then calculate
R1 from:
R1 = 1.2L/(R
L
x C9)
Add a resistor (R2 in Figure 8) to the CS- connection to
minimize input offset error. Calculate the value of R2 as
follows:
• When V
OUT
≥ 2.4V:
• When V
OUT
< 2.4V:
R
AxR
A
RxA
k
ILIM
2
15 1
15
10
32
1
=
µ
µ+
µ
Ω
⎛
⎝
⎜
⎞
⎠
⎟
R
A
RA
k
R
A
ILIM
2
20
10
32
1
20
1
=
+
×
⎛
⎝
⎜
⎞
⎠
⎟
×µ
µ
µ
Ω
I
V
R
I
LIM
TH
L
PP
=−
−
2
R
V
A
ILIM
TH
1
75
10
=
×.
µ
R
IR R
VI R
ILIM
ILIM VALLEY FOBK
OUT ILIM FO
2
2
2
=
××
+×(
BBK VALLEY
R−
()
)
R
PV
IP
FOBK
FB OUT
ILIM FB
=
×
×−
()
2
1
MAX8655
LX
R
ILIM2
R
FOBK
ILIM2
OUT
Figure 7. ILIM2 Resistor Connections
MAX8655
LX
R2
R1
L1
C9
C10
CS+
CS-
V
OUT
C11
Figure 8. Current Sense Using the Inductor’s DC Resistance