Rainbow Electronics MAX5929 - Manual (Page 18)

MAX5927/MAX5929
Sense Resistor
The slow-comparator threshold voltage is adjustable
from 25mV to 100mV. Select a sense resistor that caus-
es a drop equal to the slow-comparator threshold volt-
age at a current level above the maximum normal
operating current. Typically, set the overload current at
1.2 to 1.5 times the full load current. The fast-compara-
tor threshold is two times the slow-comparator thresh-
old in normal operating mode. Choose the sense
resistor power rating to be greater than or equal to 2 x
(I
OVERLOAD ) x V SC,TH . Table 7 lists some recommend-
ed sense resistor manufacturers.
Slow-Comparator Threshold, R LIM (MAX5927)
The slow-comparator threshold voltage is adjustable
from 25mV to 100mV, allowing designers to fine-tune
the current-limit threshold for use with standard-value
sense resistors. Low slow-comparator thresholds allow
for increased efficiency by reducing the power dissi-
pated by the sense resistor. Furthermore, the low 25mV
slow-comparator threshold is beneficial when operating
with supply rails down to 1V because it allows a small
percentage of the overall output voltage to be used for
current sensing. The VariableSpeed/BiLevel fault pro-
tection feature offers inherent system immunity against
load transients and noise. This allows the slow-com-
parator threshold to be set close to the maximum nor-
mal operating level without experiencing nuisance
faults. To adjust the slow-comparator threshold, calcu-
late R
LIM as follows:
where V
TH
is the desired slow-comparator threshold
voltage. Shorting LIM_ to GND sets V TH to 25mV. Do
not leave LIM_ open.
Setting the Startup Period, R TIM
The startup period (t START ) is adjustable from 0.4ms to
50ms. The adjustable startup period feature allows sys-
tems to be customized for MOSFET gate capacitance
and board capacitance (C
BOARD ). The startup period
is adjusted with a resistor connected from TIM to GND
(R
TIM
). R
TIM
must be between 4k Ω and 500kΩ. The
startup period has a default value of 9ms when TIM is
left floating. Calculate R
TIM with the following equation:
where t START is the desired startup period.
R
t
pF
TIM
START =
× 128 800
R
V mV
A
LIM
TH =
−
µ
25
7 5 .
Low-Voltage, Quad, Hot-Swap
Controllers/Power Sequencers
18 ______________________________________________________________________________________
PART NUMBER MANUFACTURER DESCRIPTION
IRF7413 11m Ω, 8-pin SO, 30V
IRF7401 22m Ω, 8-pin SO, 20V
IRL3502S
International Rectifier
6m Ω, D 2 PAK, 20V
MMSF3300 20m Ω, 8-pin SO, 30V
MMSF5N02H 30m Ω, 8-pin SO, 20V
MTB60N05H
Motorola
14m Ω, D 2 PAK, 50V
FDS6670A 10m Ω, 8-pin SO, 30V
ND8426A 13.5m Ω, 8-pin SO, 20V
FDB8030L
Fairchild
4.5m Ω, D 2 PAK, 30V
Table 6. Recommended N-Channel MOSFETs
COMPONENT MANUFACTURER PHONE WEBSITE
Dale-Vishay 402-562-3131 www.vishay.com
Sense Resistors
IRC 704-264-8861 www.irctt.com
International Rectifier 310-233-3331 www.irf.com
Fairchild 888-522-5372 www.fairchildsemi.com MOSFETs
Motorola 602-224-3576 www.mot-sps.com/ppd
Table 7. Component Manufacturers