SS6577 datasheet(9/14 Pages) SSC

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SS6577

Rev.1.02 3/26/2004

MAX

100mV

in the voltage of VFB pin. Thus the ITH/RUN voltage

remains increasing until the average inductor cur-

rent matches new load current. While the high-side

MOSFET turns off, the low-side MOSFET is turned

on to recharge bootstrap capacitor C3.

Inductor Selection

With the high operating frequency of 500KHz,

smaller inductor values are possible. In general,

operating at high frequency will cause low efficiency

because of large MOSFET switching loss. Thus the

effect of inductor value on ripple current and low

current operation must be considered as well.

Main control loop is shutdown when ITH/RUN goes

below 0.8V. When ITH/RUN pulled up to 0.8V or up

by error amplifier, main control loop is enabled.

Low Current Operation

During heavy load current operation, the SS6577 op-

erates in PWM mode with a frequency of 500KHz.

Decreasing of the current will cause a drop in

ITH/RUN below 1.33V so that the SS6577 enters PFM

mode operation for better efficiency. If the voltage

across RS does not exceed the offset of current

comparator within a cycle, then the high-side and

internal MOSFETs will disable until ITH/RUN goes

over 1.33V.

The inductor value has a direct influence on ripple

current ( Δ IL), which decreases with high induc-

tance and increases with high VIN or VOUT:













−

OUT

∆I

VD is the drop voltage of the output Schottky diode.

Accepting a large value ofΔIL allows the use of low

inductance, but yields high output ripple voltage

and large core loss. The inductor value also has an

effect on low current operation. Low inductor value

causes the PFM operation to begin at high load cur-

rent. The efficiency of the circuit decreases at the

beginning of low current operation. Generally

speaking, low inductance in PFM mode will cause

the efficiency to decrease.

Component Selection

The SS6

577 can be used in many switching regulator

applications, such as step-down, step-up, SEPIC

and positive-to-negative converters. Among these

step-down converter is the most common applica-

tion. External component selection, beginning with

selecting RS, depends on load requirement of the

application. Once RS is decided, the choice of in-

ductor, which is followed by selecting power

MOSFET and diode, can be easily chosen. Finally,

CIN and COUT can be determined.

Power MOSFET Selection

For an application of SS65577, an external N-

channel power MOSFET, used as the high-side

switch, must be properly selected. To prevent

MOSFET damage during high input voltage

operation, attention should be given to the BVDSS

specification of the MOSFET.

RS Selection

The choice of RS has substantial connection with

required output current. The threshold voltage of

current comparator decides peak inductor current,

which yields a maximum average output current

(IMAX), and the peak current is less than half of the

peak-to-peak ripple current, ΔIL.

Other important selection criteria for the power

MOSFET include the “ON” resistance RDS(ON), in-

put voltage and maximum output current.

Allowing a margin for variation of the SS6577, external

components can be calculated as:

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