© 2008 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN2108 • Rev. 1.0.2

10

Circuit Description

Initialization

the IC checks for an open or shorted FB pin before

releasing the internal soft-start ramp (SS).

If R1 is open (Figure 1), the error amplifier output

(COMP) is forced LOW and no pulses are generated.

After the SS ramp times out (T1.0), an under-voltage

latched fault occurs.

≤ 1 KΩ, the

internal SS ramp is not released and the regulator does

not start.

Bias Supply

The FAN2108 requires a 5 V supply rail to bias the IC

and provide gate-drive energy. Connect a

≥ 1.0 µf X5R

or X7R decoupling capacitor between VCC and PGND.

supply current is frequency and voltage dependent.

)]

128

(

)

013

.

0

227

5

[(

58

.

4

)

(

−

•

+

−

+

=

f

V

I

CC

mA

CC

(1)

where frequency (f) is expressed in KHz.

Enable

FAN2108 has an internal pull-up to enable pin so that

small capacitor across EN and AGND delays the rate of

voltage rise on the EN pin. EN pin also serves for the

restart whenever a fault occurs (refer to the Auto-Restart

section)

. For applications where sequencing is required,

external control, as shown in Figure 20.

Figure 20. Enabling with External Control

Setting the Frequency

Oscillator frequency is determined by an external resistor,

Resistance is calculated by:

65

135

)

/

10

)

(

−

=

Ω

f

R

K

T

(2)

Ω and frequency (f) is in KHz.

Soft-Start

Once internal SS ramp has charged to 0.8 V (T0.8), the

output voltage is in regulation. Until SS ramp reaches

1.0 V (T1.0), the fault latch is inhibited.

To avoid skipping the soft-start cycle, it is necessary to

Soft-start time is a function of oscillator frequency.

SS

1.35V

FB

EN

0.8V

T0.8

T1.0

3200 CLKs

4000 CLKs

Fault

Latch

Enable

0.8V

1.0V

2400 CLKs

Figure 21. Soft-Start Timing Diagram

The regulator does not allow the low-side MOSFET to

operate in full synchronous rectification mode until

helps the regulator to start on a pre-biased output and

ensures that inductor current does not "ratchet" up

during the soft-start cycle.

resets the IC.

Setting the Output Voltage

The output voltage of the regulator can be set from

The internal reference is 0.8 V with 650 nA, sourced

from the FB pin to ensure that, if the pin is open, the

regulator does not start.

The external resistor divider is calculated using:

nA

R

V

V

R

V

OUT

BIAS

650

1

8

.

0

8

.

0

+

−

=

(3)