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FSL116LR

Green Mode Fairchild Power Switch (FPS™)

Fairchild Semiconductor 

Output-Short Protection (OSP)

If the output is shorted, steep current with extremely

high di/dt can flow through the SenseFET during the

LEB time. Such a steep current brings high-voltage

stress on the drain of SenseFET when turned off. To

protect the device from such an abnormal condition,

OSP detects VFB and SenseFET turn-on time. When the

VFB is higher than 1.6V and the SenseFET turn-on time

is lower than 1.0µs, the FPS recognizes this condition

as an abnormal error and shuts down PWM switching

until VCC reaches VSTART again. An abnormal condition

output is shown in Figure 20.

continues, but the current limit is fixed internally to

minimize flux density in the transformer. The fixed

current limit is larger than that defined by VFB = VBURH

and, therefore, VFB is driven down further. Switching

continues until the feedback voltage drops below VBURL.

At this point, switching stops and the output voltages

start to drop at a rate dependent on the standby current

load. This causes the feedback voltage to rise. Once it

passes VBURH, switching resumes. The feedback voltage

then falls and the process repeats. Burst mode

alternately enables and disables switching of the

SenseFET and reduces switching loss in standby mode.

Figure 20. Output Short Waveforms (OSP)

Soft-Start

The FPS has an internal soft-start circuit that slowly

increases the feedback voltage, together with the

SenseFET current, after it starts. The typical soft-start

time is 20ms, as shown in Figure 21, where progressive

increments of the SenseFET current are allowed during

the startup phase. The pulse width to the power

switching device is progressively increased to establish

the correct working conditions for transformers,

inductors, and capacitors. The voltage on the output

capacitors is progressively increased with the intention

of smoothly establishing the required output voltage.

Soft-start helps to prevent transformer saturation and

reduce the stress on the secondary diode.

1.25ms

ILIM

16 Steps

Current Limit

Figure 22. Burst-Mode Operation

Adjusting Peak Current Limit

As shown in Figure 23, a combined 6kΩ internal

resistance is connected to the non-inverting lead on the

PWM comparator. An external resistance of Rx on the

current limit pin forms a parallel resistance with the 6kΩ

when the internal diodes are biased by the main current

source of 400µA. For example, FSL116LR has a typical

SenseFET peak current limit (ILIM) of 1.2A. ILIM can be

adjusted to 0.8A by inserting Rx between the IPK pin and

the ground. The value of the Rx can be estimated by the

following equations:

1.2A : 0.8A = 6kΩ : XkΩ

(1)

X = Rx || 6kΩ

(2)

where X is the resistance of the parallel network.

0.25ILIM

Drain

Current

t

Figure 21. Internal Soft-Start

Burst Operation

To minimize power dissipation in standby mode, the

FPS enters burst mode. As the load decreases, the

feedback voltage decreases. As shown in Figure 22, the

device automatically enters burst mode when the

feedback voltage drops below VBURH. Switching

© 2010 Fairchild Semiconductor Corporation

FSL116LR • Rev. 1.0.1

11

Figure 23. Peak Current Limit Adjustment

www.fairchildsemi.com

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