max5938 Maxim Integrated Products, Inc., max5938 Datasheet - Page 16

• Manufacturer Part Number: max5938
• Description: Max5938 -48v Hot-swap Controller With Vin Step Immunity, No Rsense, And Overvoltage Protection
• Manufacturer: Maxim Integrated Products, Inc.

normalized units, typically with a value of 1 at +25°C, it
is possible to multiply the curve by the drain voltage at
+25°C and convert the curve to drain voltage. Now
compare this curve to that of the MAX5938 normalized
tempco of the circuit-breaker threshold to make a
determination of the tracking error in mV between the
power MOSFET [I
MAX5938 [I
operating temperature range of the application.
If the tempco of the power MOSFET is greater than the
MAX5938’s, then additional margin in setting the circuit-
breaker and short-circuit voltages will be required at
higher temperatures as compared to +25°C (Figure 15).
When dissipation in the power MOSFET is expected to
lead to local temperature elevation relative to ambient
-48V Hot-Swap Controller with V
No R
Figure 14. MAX5938 Normalized Circuit Breaker
Figure 15. Circuit-Breaker Voltage Margin For High and Low Tempco Power MOSFETS
16
______________________________________________________________________________________
SENSE
CIRCUIT-BREAKER
TRIP REGION
CB_ADJ
1.6
1.4
1.2
1.0
0.8
0.6
0.4
-40
NORMALIZED MOSFET ON-RESISTANCE
(µA) x (2kΩ + R
IRF1310NS
NORMALIZED R
-15
D,MOSFET
T
vs. TEMPERATURE
A
TEMPERATURE (°C)
= +25°C
, and Overvoltage Protection
10
IRFR3910
NORMALIZED R
ON
R
35
DS(ON)
MAX5938
NORMALIZED V
x R
TEMPERATURE
1/2 x I
HIGH TEMPCO
DS(ON)
ON
CB_ADJ
60
CB_ADJ
I
D
CB
x R
DS(ON)
85
x R
∆V
] and the
)] over the
CB_ADJ
CB,MIN
conditions, it becomes imperative that the MAX5938 be
located as close as possible to the power MOSFET. The
marginal effect of temperature differences on circuit-
breaker and short-circuit voltages can be estimated
from a comparative plot such as Figure 14.
When a positive V
increase results in a voltage rise at both STEP_MON
and V
STEP_MON is above STEP
short-circuit and circuit-breaker faults. During this
STEP_MON high condition, if V
MAX5938 will immediately and very rapidly pull GATE to
V
current spiking. GATE is held low for 350µs. About 1ms
after the start of GATE pulldown, the MAX5938 begins to
ramp GATE up to turn on the MOSFET in a controlled
manner that results in ramping V
supply level (see the GATE Cycles section in Appendix
A). This occurs with the least possible disturbance to
V
is off, the voltage across the load droops slightly
depending on the load current and load storage capaci-
tance. PGOOD remains asserted throughout the V
step event.
The objective in selecting the resistor and capacitor for
the step monitor function is to ensure that the V
of all anticipated slopes and magnitudes will be proper-
ly detected and blocked, which otherwise would result
in a circuit-breaker or short-circuit fault. The following is
a brief analysis for finding the resistor and capacitor.
For a more complete analysis, see Appendix B.
EE
OUT
. This turns off the power MOSFET to avoid inrush
, although during the brief period that the MOSFET
OUT
CIRCUIT-BREAKER
TRIP REGION
Selecting a Resistor and Capacitor
∆V
CB,MIN
relative to V
IN
T
A
IN
Step Immunity,
= +25°C
step or ramp occurs, the V
EE
TH
. When the voltage at
R
OUT
DS(ON)
, the MAX5938 blocks
TEMPERATURE
OUT
for Step Monitor
LOW TEMPCO
rises above V
1/2 x I
I
D
down to the new
x R
DS(ON)
CB_ADJ
x R
CB_ADJ
IN
SC
steps
, the
IN
IN