SS6577 datasheet(10/14 Pages) SSC

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SS6577

Rev.1.02 3/26/2004

Output Diode Selection











 +

VOUT

In order not to exceed the diode ratings, it is impor-

tant to specify the diode peak current and average

power dissipation.

The feedback reference voltage 0.8V allows low

output voltages from 0.8V to input voltage. A small

capacitor at 1nF in parallel to the upper feedback

resistor is required for a stable feedback.

CIN and COUT Selection

To prevent the high voltage spike resulted from high

frequency switching, a low ESR input capacitor for

the maximum RMS current must be used. Usually

capacitors may be paralleled to meet size or height

requirements in the design.

ITH/RUN Function

The ITH/RUN pin, also as a dual-purpose pin, pro-

vides loop compensation as well as shutdown func-

tion. An internal current source at 2.5

µA charges up

the external capacitor C5. When the voltage on

ITH/RUN pin reaches 0.8V, the SS6577 begins to

operate.

The selection of COUT depends on the required ef-

fective series resistance (ESR). In general once the

ESR requirement is met, the capacitance is suitable

for filtering. The output ripple voltage (ΔVOUT) is

determined by:

330pF

24k

LL4148

µF

1.2M

VIN 4.5V~24V

ITH/RUN













∆

≈

∆

OUT

ESR

where f = operating frequency, COUT = output ca-

pacitance and ΔIL = ripple current of the inductor.

Once the ESR requirement for COUT has been met,

the RMS current rating generally far exceeds the

IRIPPLE(P-P) requirement.

Fig. 12 ITH/RUN pin interfacing

Topside MOSFET Driver Supply (C3)

External bootstrap capacitor C3 connecting to

BOOST pin supplies the gate drive voltage for high-

side MOSFET. C3 is charged from INTVCC when

SW pin is low. When the high-side MOSFET turns

on, the driver places the C3 voltage across the gate

to the source of MOSFET. It will enhance the

MOSFET and turn on the high-side switch. Then

the switch node voltage SW rises to VIN and

BOOST pin rises to VIN + INTVCC. In general,

0.1

µF is acceptable.

Over Current Protection

Over current protection occurs when the peak in-

ductor current reaches maximum current sense

threshold divided by sense resistor. The maximum

current under over current protection can be calcu-

lated by the following formula.

threshold)

sense

current

mum

150mV(Maxi

MAX

At the same time, the frequency of oscillator will be

reduced to sixteenth of original value, 500kHz. This

lower frequency allows the inductor current to

safely discharge, thereby preventing current run-

away. The frequency of oscillator will automatically

Output Voltage Programming

The typical SS6577 application circuit is shown in

figure17. A resistive divider, as in the following for-

mula, sets the output voltage.

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