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LTC4352

4352f

SOURCE

LTC4352

TO LOAD

Si7336ADP

4352 F07

VIN

GATE

OUT

0.1μF

GND

12V

CPO

Figure 7. 5V Ideal Diode with External 12V Powering CPO for

Faster Start-up and Refresh

APPLICATIONS INFORMATION

External CPO Supply

The internal charge pump takes milliseconds to charge

up the CPO pin capacitor especially during device power

up. This time can be shortened by connecting an external

supply to the CPO pin. A series resistor is needed to limit

the current into the internal clamp between the CPO and

SOURCE pins. The CPO supply should also be higher than

the main input supply to meet the gate drive requirements

of the MOSFET. Figure 7 shows such a 5V ideal diode ap-

plication, where a 12V supply is connected to the CPO pin

through a 1k resistor. The 1k limits the current into the CPO

pin to 5.3mA, when the SOURCE pin is grounded.

Input Transient Protection

When the capacitances at the input and output are very

small, rapid changes in current can cause transients that

exceed the 24V Absolute Maximum Rating of the VIN and

OUT pins. In ORing applications using a single MOSFET, one

surge suppressor connected from OUT to ground clamps all

the inputs. In the absence of a surge suppressor, an output

capacitance of 10μF is sufﬁcient in most applications to

prevent the transient from exceeding 24V. Back-to-back

MOSFET applications, depending on voltage levels, may

require a surge suppressor on each supply input.

Design Example

The following design example demonstrates the calcula-

tions involved for selecting components in a 12V system

with 10A maximum load current (see Figure 1).

First, calculate the RDS(ON) of the MOSFET to achieve the

desired forward drop at full load. Assuming a VFWD of

50mV (which is comfortably below the 200mV minimum

open MOSFET fault threshold):

DS ON

() ≤

FWD

LOAD

50mV

10A

= 5mΩ

The Si7336ADP offers a good solution, in a SO-8 sized

package, with a maximum RDS(ON) of 4mΩ and BVDSS

of 30V. The maximum power dissipation in the MOSFET

is:

P = I2LOAD • RDS(ON) = (10A)2 • 4mΩ = 0.4W

With a maximum steady-state thermal resistance, θJA,

of 65°C/W, 0.4W causes a modest 26°C rise in junction

temperature of the Si7336ADP above the ambient.

The input capacitance, CISS, of the Si7336ADP is about

6500pF. Slightly exceeding the 10x recommendation, a

0.1μF capacitor is selected for C2.

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