Datasheet

network (R4 and R5) used for setting the adjustable
current limit as shown in Figure 7.
The following is a procedure for calculating the value of
R4, R5, and R6:
1) Calculate the voltage, V
ILIM(NOM)
, required at ILIM
when the output voltage is at nominal:
2) Pick a percentage of foldback, PFB, from 15%
to 40%.
3) Calculate the voltage, V
ILIM(0V)
, when the output is
shorted (0V):
4) The value for R4 can be calculated as:
5) The parallel combination of R5 and R6, denoted
R56, is calculated as:
6) Then R6 can be calculated as:
7) Then R5 is calculated as:
Boost-Supply Diode and
Capacitor Selection (Buck)
A low-current Schottky diode, such as the CMDSH-3
from Central Semiconductor, works well for most appli-
cations. Do not use large-power diodes, because high-
er junction capacitance can charge up the voltage at
BST to the LX voltage and this exceeds the absolute
maximum rating of 6V. The boost capacitor should be
0.1µF to 4.7µF, depending on the input and output volt-
ages, external components, and PC board layout. The
boost capacitance should be as large as possible to
prevent it from charging to excessive voltage, but small
enough to adequately charge during the minimum low-
side MOSFET conduction time, which happens at maxi-
mum operating duty cycle (this occurs at minimum
input voltage). In addition, ensure that the boost capac-
itor does not discharge to below the minimum gate-to-
source voltage required to keep the high-side MOSFET
fully enhanced for lowest on-resistance. This minimum
gate-to-source voltage (V
GS(MIN)
) is determined by:
where V
DD
is 5V, Q
G
is the total gate charge of the
high-side MOSFET, and C
BOOST
is the boost-capacitor
value where C
BOOST
is C7 in Figure 8.
VVx
Q
C
GS MIN DD
G
BOOST
()
=
R
RR
RR
5
656
656
=
×
-
R
VRR
VV V R
VVR
OUT
OUT ILIM NOM ILIM V
ILIM NOM ILIM V
6
6
56
0
0
=
××
()
()
×
−
()
×
()










() ()
() ()
4 5
-- 4-
R
V
A
R56
2
10
=






µ
- 4
R
VV
A
ILIM V
4
2
10
0
=
()
-
µ
VPV
ILIM V FB ILIM NOM() ( ) 0
=×
VI
LIR
R
ILIM NOM LOAD MAX
DS ON Q
() ()
()
=× ×






×
10 1
2
2
-
MAX8632
Integrated DDR Power-Supply Solution for
Desktops, Notebooks, and Graphic Cards
______________________________________________________________________________________ 23
Figure 7. Foldback Current Limit
MAX8632
REF
ILIM
GND
C
REF
R4
R6
R5
V
OUT