MAX8553 Maxim, MAX8553 Datasheet - Page 20

• Manufacturer Part Number: MAX8553
• Description: 4.5V to 28V Input / Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
• Manufacturer: Maxim

The adjustable current limit accommodates MOSFETs
with a wide range of on-resistance values. The current-
limit threshold is adjusted with an external resistor con-
nected from ILIM to GND (R
range is 50mV to 200mV measured across the low-side
MOSFET. The value of R
lowing formula:
where I
the on-resistance of the low-side MOSFET. To avoid
reaching the current at a lower current than expected,
use the maximum value for R
tion temperature. Refer to the MOSFET manufacturer’s
data sheet for maximum values.
Foldback current limit is used to reduce power dissipa-
tion during overload and short-circuit conditions. This is
accomplished by lowering the current-limit threshold as
the output voltage drops due to the overload.
To use foldback current limit, connect a resistor
(R
(R
and R
First, select the percentage of foldback, P
cent corresponds to the current limit when V
zero divided by the current limit when V
nominal voltage. Typical values range from 15% to
30%. To solve for the resistor values, use the following
equations:
If R
side MOSFET with lower R
combination of both for the best compromise of cost,
efficiency, and lower short-circuit power dissipation.
4.5V to 28V Input, Synchronous PWM Buck Controllers
for DDR Termination and Point-of-Load Applications
20
FOBK
ILIM
R
ILIM
ILIM
______________________________________________________________________________________
FOBK
) from ILIM to GND
VALLEY
) from ILIM to the output, and connect a resistor
=
results in a negative number, select another low-
10
V
R
OUT
are calculated as follows:
ILIM
×
is the valley current limit and R
-
R
R
(
=
FOBK
DS ON
10
10
(
×
)
=
×
R
×
Setting the Current Limit
DS ON
ILIM
5
P
I
I
(Figure
VALLEY
VALLEY
(
FB
µ
DS(ON)
5µ
A
)
is calculated using the fol-
(
×
A
1
DS(ON)
×
Foldback Current Limit
Constant Current Limit
ILIM_
-
V
×
I
5). The values of R
OUT
VALLEY
(
P
1
×
or increase P
FB
-
). The adjustment
R
P
)
FB
DS ON
at elevated junc-
)
×
(
OUT
×
(
FB
1
R
)
OUT
-
FOBK
. This per-
equals its
P
DS(ON)
FB
FB
equals
)
)
or a
ILIM
is
The switching frequency of the MAX8553/MAX8554 can
be lowered from the value set by FSEL by adding a
resistor voltage-divider to EN/HSD (HSD) as shown in
Figure
measures on EN/HSD (HSD), which increases the on-
time. The switching frequency with the added resistor-
divider is calculated as follows:
where K = 1.7µs and N is given in Tables 1 and 2. To set
the frequency, select a value for R2 between 10kΩ and
100kΩ, then calculate R1 from the following equation:
With the minimum input voltage, make sure that the
voltage present at EN/HSD (HSD) is greater than 1.5V
when the resistor-divider is used:
Figure
Resistors, R ILIM and R FOBK
R
ILIM
R
5. Setting the Foldback Current Limit with Two
FOBK
6. This voltage-divider lowers the voltage the IC
Adjusting the Switching Frequency
ILIM
f
S
R
V
MAX8553
1
IN MIN
≅
R
=
(
1
K
K N
+
×
1
)
1
×
PGND
R
×
DH
N
DL
LX
2
R
×
×
2
R
f
S
2
>
R
-
1
1 5
R
R
.
+
2
2
V
R
2
V
OUT