FSBH0170ANY datasheet(13/16 Pages) FAIRCHILD

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FSBH0F70A, FSBH0170/A, FSBH0270/A, FSBH0370 • Rev. 1.0.3

Brown-In Function of FSBH0x70A

The VIN pin functions are disabled from FSBH0x70A,

but FSBH0x70A has brown-in protection in the VDD pin.

There is a discharge current internal from VDD to ground

during startup. The HV source current must be larger

than IDD-ST to charge the capacitor of VDD. Therefore, the

brown-in level can be determined by RHV according to

the equation:

212

DD ST

−

(4)

Green-Mode Operation

The FSBH-series uses feedback voltage (VFB) as an

indicator of the output load and modulates the PWM

frequency, as shown in Figure 30, such that the

switching frequency decreases as load decreases. In

heavy load conditions, the switching frequency is

100kHz. Once VFB decreases below VFB-N (2.5V), the

PWM frequency starts to linearly decrease from 100kHz

to 18kHz for reducing switching losses. As VFB

decreases below VFB-G (2.0V), the switching frequency is

fixed at 18kHz and FSBH-series enters “deep” green

mode to reduce the standby power consumption.

Figure 30.

PWM Frequency

As VFB decreases below VFB-ZDC (1.6V), FSBH-series

enters into burst-mode operation. When VFB drops

below VFB-ZDC, FSBH-series stops switching and the

output voltage starts to drop, which causes the

feedback voltage to rise. Once VFB rises above VFB-ZDC,

switching resumes. Burst mode alternately enables and

disables switching, thereby reducing switching loss to

improve power saving, as shown in Figure 31.

VFB

(1.6V)

IDrain

Switching

Disabled

VFB.ZDC

Switching

Disabled

Figure 31. Burst-Mode Operation

H/L Line Over-Power Compensation

To limit the output power of the converter constantly,

high/low line over-power compensation is included.

Sensing the converter input voltage through the VIN pin,

the high/low line compensation function generates a

relative peak-current-limit threshold voltage for constant

power control, as shown in Figure 32.

Figure 32.

Constant Power Control

Protections

The FSBH-series provides full protection functions to

prevent the power supply and the load from being

damaged. The protection features include:

Latch/Auto-Recovery Function

Besides the brownout protection and high/low line over-

power compensation, the FSBH0170/0270/0370 has

additional protections via the VIN pin, such as pull-

HIGH latch and pull-LOW auto-recovery that depends

on the application. As shown in Figure 33, VIN level is

higher than 4.7V, FSBH-series is latched until the VDD is

discharged. FSBH-series is auto-recovery when the VIN

level is lower than 0.3V.

Figure 33.

VIN Pin Function

Open-Loop / Overload Protection (OLP)

When the upper branch of the voltage divider for the

shunt regulator (KA431 shown) is broken, as shown in

Figure 34, or over current or output short occurs, there

is no current flowing through the opto-coupler transistor,

which pulls the feedback voltage up to 6V.

When feedback voltage is above 4.6V for longer than

56ms, OLP is triggered. This protection is also triggered

when the SMPS output drops below the nominal value

longer than 56ms due to the overload condition.

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