1V to 13.2V, n-Channel Hot-Swap Controllers
Require No Sense Resistor
______________________________________________________________________________________   17
Circuit-Breaker Temperature Coefficient
In applications where the external MOSFETs on-resis-
tance is used as a sense resistor to determine overcur-
rent conditions, a 3300ppm/癈 temperature coefficient
is desirable to compensate for the R
DS(ON)
tempera-
ture coefficient. Use the MAX5926s TC input to select
the circuit-breaker programming currents temperature
coefficient, TC
ICB
(see Table 2). The MAX5924 temper-
ature coefficient is preset to 0ppm/癈, and the
MAX5925s is preset to 3300ppm/癈.
Setting TC
ICB
to 3300ppm/癈 allows the circuit-breaker
threshold to track and compensate for the increase in the
MOSFETs R
DS(ON)
with increasing temperature. Most
MOSFETs have a temperature coefficient within a
3000ppm/癈 to 7000ppm/癈 range. Refer to the MOSFET
data sheet for a device-specific temperature coefficent.
R
DS(ON)
and I
CB
are temperature dependent, and can
therefore be expressed as functions of temperature. At
a given temperature, the MAX5925/MAX5926 indicate
an overcurrent condition when:
I
TRIPSLOW
x R
DS(ON)
(T) e I
CB
(T) x R
CB
+ |V
CB,OS
|
where V
CB,OS
is the worst-case offset voltage. Figure 14
graphically portrays operating conditions for a MOSFET
with a 4500ppm/癈 temperature coefficient.
Applications Information
Component Selection
n-Channel MOSFET
Most circuit component values may be calculated with
the aid of the MAX5924MAX5926. The "Design calcula-
tor for choosing component values" software can be
downloaded from the MAX5924MAX5926 Quickview on
the Maxim website.
Select the external n-channel MOSFET according to the
applications current and voltage level. Table 3 lists some
recommended components. Choose the MOSFETs
on-resistance, R
DS(ON)
, low enough to have a minimum
voltage drop at full load to limit the MOSFET power dis-
sipation. High R
DS(ON)
can cause undesired power
loss and output ripple if the board has pulsing loads or
triggers an external undervoltage reset monitor at full
load. Determine the device power-rating requirement to
accommodate a short circuit on the board at startup
with the device configured in autoretry mode.
Using the MAX5924/MAX5925/MAX5926 in latched mode
allows the consideration of MOSFETs with higher R
DS(ON)
and lower power ratings. A MOSFET can typically with-
stand single-shot pulses with higher dissipation than the
specified package rating. Low MOSFET gate capaci-
tance is not necessary since the inrush current limiting is
achieved by limiting the gate dv/dt. Table 4 lists some
recommended manufacturers and components.
Be sure to select a MOSFET with an appropriate gate
drive (see the Typical Operating Characteristics).
Typically, for V
CC
less than 3V, select a 2.5V V
GS
MOSFET.
Table 2. Programming the Temperature
Coefficient (MAX5926)
TC
TC
ICB
(ppm/癈)
High
0
Low
3300
Table 3. Suggested External MOSFETs
APPLICATION
CURRENT (A)
PART
DESCRIPTION
1
International Rectifier
IRF7401
SO-8
2
Siliconix Si4378DY
SO-8
5
Siliconix SUD40N02-06
DPAK
10
Siliconix SUB85N02-03
D2PAK
TEMPERATURE (癈)
85
60
35
10
-15
25
30
35
40
45
50
20
-40
110
V
S
= V
CC
= 13.2V, R
CB
= 672&, I
TRIPSLOW
= 5A,
R
DS(ON)
(25) = 6.5m&
CIRCUIT-BREAKER TRIP REGION
(V
SENSE
e V
CB
)
V
CB
= I
CB
(T) x R
CB
+ V
CB,OS
(3300ppm/癈)
V
SENSE
= R
DS(ON)
(T) x I
LOAD(MAX)
(4500ppm/癈)
Figure 14. Circuit-Breaker Trip Point and Current-Sense
Voltage vs. Temperature