Application Information

Input Capacitor

This IntelliMAX™ switch doesn’t require an input

capacitor. To reduce device inrush current, a 0.1 µF

ceramic capacitor, CIN, is recommended close to the

VIN pin. A higher value of CIN can be used to reduce the

voltage drop experienced as the switch is turned on into

a large capacitive load.

Output Capacitor

While this switch works without an output capacitor: if

parasitic board inductance forces VOUT below GND

when switching off; a 0.1 µF capacitor, COUT, should be

placed between VOUT and GND.

Fall Time

Device output fall time can be calculated based on RC

constant of the external components as follows:

tF RL CL 2.2

(1)

where tF is 90% to 10% fall time, RL is output load,

and CL is output capacitor.

The same equation works for a device with a pull-down

output resistor. RL is replaced by a parallel connected

pull-down and an external output resistor combination as:

tF

RL

RL

RPD

RPD

CL

2.2

(2)

where tF is 90% to 10% fall time, RL is output load,

RPD=65 is output pull-down resistor, and CL is the

output capacitor.

Resistive Output Load

If resistive output load is missing, the IntelliMAX switch

without a pull-down output resistor does not discharge

the output voltage. Output voltage drop depends, in that

case, mainly on external device leaks.

Application Specifics

At maximum operational voltage (VIN=5.5 V), device

inrush current might be higher than expected. Spike

current should be taken into account if VIN>5 V and the

output capacitor is much larger than the input capacitor.

Input current can be calculated as:

IIN(t)

VOUT (t)

RLOAD

(CLOAD

CIN

)

dVOUT

dt

(t)

(3)

where switch and wire resistances are neglected and

capacitors are assumed ideal.

Estimating VOUT(t)=VIN/10 and using experimental

formula for slew rate (dVOUT(t)/dt), spike current can be

written as:

max IIN

VIN

10RLOAD

CLOAD

CIN0.05VIN

0.255

(4)

where supply voltage VIN is in volts, capacitances are

in micro farads, and resistance is in ohms.

Example: If VIN=5.5V, CLOAD=100 µF, CIN=10 µF, and

RLOAD=50 ; calculate the spike current by:

max(

IIN)

5.5

1050

(100

10)(0.055.5

0.255 )A

1.8A

(5)

Maximum spike current is 1.8 A, while average ramp-

up current is:

IIN(t)

VOUT (t)

RLOAD

(CLOAD

CIN )

dVIN(t)

dt

2.75 / 50 100 0.0022 0.275 A

(6)

Output Discharge

FPF1039 contains a 65 Ω on-chip pull-down resistor for

quick output discharge. The resistor is activated when

the switch is turned off.

Recommended Layout

For best thermal performance and minimal inductance

and parasitic effects, it is recommended to keep input

and output traces short and capacitors as close to the

device as possible. Figure 34 is a recommended layout

for this device to achieve optimum performance.

Vin Vout

VIN FPF1039

CIN OFF ON ON

GND

+

CLOAD

RLOAD

Figure 33. Device Setup

© 2010 Fairchild Semiconductor Corporation

FPF1039 • Rev. 1.5

Figure 34. Recommended Land Pattern, Layout

www.fairchildsemi.com

11