Smallest, Most Reliable, 12V, Infiniband-
Compliant Hot-Swap Controller
Startup Into Load
The MAX8533 is intended to be used in a circuit where
no load is applied until the POK signal is enabled. In an
application where the load is applied during the output-
voltage ramp up, the R
DS(ON)
of the MOSFET is higher
and the power dissipated by the MOSFET is larger.
Repeated, rapid hot-swaps into a load can create suffi-
cient heat to exceed the power-dissipation limits of the
package causing failure of the MOSFET.
MAX8533
IN
GATE
I
SET
I
REG
I0µA
CHARGE
PUMP
OVERCURRENT
COMPARATOR
IN
CURRENT-REGULATION
CONTROL LOGIC
5V LINEAR
REGULATOR
2V
REFERENCE
(REF)
UVLO
COMPARATOR
SEVERE
OVERCURRENT
COMPARATOR
BIAS
IN
7.5kΩ
Applications Information
Setting the Turn-On Ramp Rate
The MOSFET turn-on ramp rate is determined by the
capacitor (C2) at GATE (Figure 2). An internal 10µA
current source charges C2 to bring GATE high. The
soft-start rate is determined as follows:
t
SS
= C2 x (V
IN
+ 5)/(10 x 10
-6
)
t
SS
may be longer than expected in the application
depending on the gate capacitance of the external
MOSFET. The necessary gate voltage is provided by
an on-board charge pump that boosts the voltage at
GATE to (V
IN
+ 5V).
20µA
IN
CURRENT-
REGULATION
TIMER
OVP
COMPARATOR
2V
POK
COMPARATOR
2V
CTIM
OVP
OUT
RET
EN
LPEN
20µA
5V
CLAMP
External Power MOSFET Selection
Select the N-channel MOSFET according to the appli-
cation’s current requirements. Table 2 lists some rec-
ommended components. The MOSFET’s on-resistance
(R
DS(ON)
) should be chosen low enough to have a min-
imal voltage drop at full load to limit the MOSFET power
dissipation. High R
DS(ON)
can cause high output ripple
if the board has pulsing loads, or trigger an external
5V
CLAMP
MAX8533
20µA
POK
IN
N
Figure 1. MAX8533 Functional Diagram
BACKPLANE
V
IN
100µF
10µF
INFINIBAND
MODULE
10V TO 14V
R1
10mΩ
N1
R2
20Ω
C2
0.1µF
ISET
GATE
OUT
V
IN
R5
31.6kΩ
V
VB_OUT
C1
470µF
R3
3.09kΩ
VBxEN
LPEN
VB_RET
EN
LPEN
C4
0.22µF
CTIM
C3
0.01µF
IN
MAX8533
RET
POK
OVP
R4
4.99kΩ
R6
100kΩ
POK
Figure 2. Typical 50W Applications Circuit
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