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LTC4236

4236f

For more information www.linear.com/LTC4236

applicaTions inForMaTion

Supply Transient Protection

When the capacitances at the input and output are very

small, rapid changes in current during input or output

short-circuit events can cause transients that exceed the

24V absolute maximum ratings of the IN and OUT pins.

To minimize such spikes, use wider traces or heavier

trace plating to reduce the power trace inductance. Also,

bypass locally with a 10µF electrolytic and 0.1µF ceramic,

or alternatively clamp the input with a transient voltage

suppressor (Z1, Z2). A 100Ω, 0.1µF snubber damps the

response and eliminates ringing (See Figure 9).

Design Example

As a design example for selecting components, consider a

12V system with a 7A maximum load current for the two

supplies (see Figure 1).

First, select the appropriate value of the current sense

resistor RSforthe12Vsupply.Calculatethesenseresistor

value based on the maximum load current ILOAD(MAX) and

the lower limit for the current limit sense voltage threshold

∆VSENSE(TH)(MIN).

RS =

ΔVSENSE(TH)(MIN)

ILOAD(MAX)

22.5mV

= 3.2mΩ

Choose a 3mΩ sense resistor with a 1% tolerance.

Next, calculate the RDS(ON) of the ideal diode MOSFET to

achievethedesiredforwarddropatmaximumload.Assum-

ing a forward drop, ∆VFWD of 30mV across the MOSFET:

RDS(ON) ≤

ΔVFWD

ILOAD(MAX)

30mV

= 4.2mΩ

The SiR158DP offers a good choice with a maximum

RDS(ON) of 1.8mΩ at VGS = 10V. The input capacitance

CISS of the SiR158DP is about 4980pF. Slightly exceeding

the 10× recommendation, a 0.1µF capacitor is selected

for C2 and C3 at the CPO pins.

Next, verify that the thermal ratings of the selected Hot

Swap MOSFET are not exceeded during power-up or an

overcurrent fault.

Assuming the MOSFET dissipates power due to inrush

current charging the load capacitor CL at power-up, the

energy dissipated in the MOSFET is the same as the energy

stored in the load capacitor, and is given by:

ECL =

• CL • VIN2

For CL = 680µF, the time it takes to charge up CL is cal-

culated as:

tCHARGE =

CL • VIN

IINRUSH

680µF • 12V

= 8ms

The inrush current is set to 1A by adding capacitance CHG

at the gate of the Hot Swap MOSFET.

CHG =

CL • IHGATE(UP)

IINRUSH

680µF • 10µA

= 6.8nF

Choose a practical value of 10nF for CHG.

TheaveragepowerdissipatedintheMOSFETiscalculatedas:

PAVG =

ECL

tCHARGE

•

680µF • 12V

(

)

8ms

= 6W

The MOSFET selected must be able to tolerate 6W for 8ms

duringpower-up.TheSOAcurvesoftheSiR158DPprovide

45W (1.5A at 30V) for 100ms. This is sufficient to satisfy

the requirement. The increase in junction temperature due

to the power dissipated in the MOSFET is ∆T = PAVG • ZthJC

where ZthJC is the junction-to-case thermal impedance.

Under this condition, the SiR158DP data sheet indicates

that the junction temperature will increase by 3°C using

ZthJC = 0.5°C/W (single pulse).

Next, the power dissipated in the MOSFET during an

overcurrent fault must be safely limited. The fault timer

capacitor (CFT) is used to prevent power dissipation in

the MOSFET from exceeding the SOA rating during active

current limit. A good way to determine a suitable value

for CFT is to superimpose the foldback current limit profile

shown in the Typical Performance Characteristics on the

MOSFET data sheet’s SOA curves.

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