LTC4242_15 Datasheet, PDF (19/24 Pages) Linear Technology – Dual Slot Hot Swap Controller for PCI Express

LTC4242
APPLICATIO S I FOR ATIO
In system board applications, large bypass capacitors
(≥10µF) are recommended at each of the system input
supplies to minimize supply glitches as a result of large
inrush or fault currents.
It is important to put C1, the bypass capacitor for the V_CC
pin as close as possible between the V_CC and GND pins.
Design Example
Consider a PCI Express Hot Swap application example
with the following power supply requirements:
Table 1. PCI Express Power Supply Requirements
MAXIMUM SUPPLY
SUPPLY VOLTAGE
CURRENT
MAXIMUM LOAD
CAPACITANCE
12V
3.3V
3.3V_AUX
5.5A
3.0A
375mA
2000µF
1000µF
150µF
1. Select an R_SENSE value for each supply. Calculate the
R_SENSE value based on the maximum load current and the
lower circuit breaker threshold limit, ΔV_{SENSE(CB)(MIN)}. In
a PCI Express connector, five pins are allocated for the
12V supply, three pins for the 3.3V supply and one pin for
3.3V_AUX. The current rating of a connector pin is 1.1A. If
a 1% tolerance is assumed for the sense resistors, then
the following values of resistances should suffice:
Table 2. Sense Resistance Values
VOLTAGE SUPPLY
12V
3.3V
RSENSE (1%)
8mΩ
13mΩ
ITRIP(MIN)
5.6A
3.4A
ITRIP(MAX)
6.9A
4.3A
2. Assume no load current at start-up and the inrush current
charges the load capacitance. Compute gate capacitance
with:
C_GATE
=
I_GATE(UP)
V_OUT
•
t₁
(2)
t₁ is the time to charge up the load capacitor.
With I_{GATE(UP)(MAX)} = 13µA and t₁ = 10ms:
a. For 12V Supply, C_GATE = 11nF
b. For 3.3V Supply, C_GATE = 39nF
So a value of 15nF and 47nF (±10%) should suffice for
the 12V and 3.3V supplies respectively. The worst-case
t₁ and inrush currents are tabulated in Table 3.
Table 3. Worst-Case t₁ and Inrush Current
VOLTAGE SUPPLY
t_1(MIN)
t_1(MAX)
12V
13ms
40ms
3.3V
11ms
34ms
MAX I_INRUSH
2.4A
0.4A
For the internal switch, the slew rate (SR) at the 3.3V_AUX
supply output is limited to 1.7V/ms max. The inrush cur-
rent can then be calculated according to:
I_INRUSH(MAX) = C_LOAD • SR_MAX
(3)
The inrush current must be lower than 385mA (I_CBAUX(MIN))
for proper start-up. Assuming a tolerance of 30% for the
load capacitance, the value of C_LOAD should not exceed
170µF.
3. Next is the selection of MOSFETs for the 12V and 3.3V
main input supplies. The Si7336ADP’s on resistance is less
than 4mΩ at V_GS = 4.5V, 25°C and it is a good choice for
3.3V and 12V main supplies.
Since the maximum load for the 3.3V supply is 3A, the
MOSFET may dissipate up to 36mW. The Si7336ADP
has a maximum junction-to-ambient thermal resistance
of 50°C/W. This gives a junction temperature of 51.8°C
when operating at a case temperature of 50°C. Accord-
ing to the Si7336ADP’s Normalized On-Resistance vs
Junction Temperature curve, the device’s on resistance
can be expected to increase by about 12% over its room
temperature value. Recalculation for steady-state R_ON
and junction temperature yield approximately 4.5mΩ
and 52°C, respectively. The voltage drop across the 3.3V
sense resistor and series MOSFET at 3A and at 50°C PCB
temperature is less than 53mV.
The MOSFET dissipates power during inrush charging of
the output load capacitor. Assuming no load current, the
MOSFET’s dissipated power equals the final load capaci-
tor stored energy. Therefore, average MOSFET dissipated
power is:
P_ON
=
C_L
•
2
V_O2UT
• t₁
(4)
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