Oscillator Frequency

The SYNC input controls the oscillator frequency.

operation; connecting it to REF selects 300kHz opera-

tion. SYNC can also be driven with an external 240kHz

to 350kHz CMOS/TTL source to synchronize the inter-

nal oscillator. Normally, 300kHz operation is chosen to

minimize the inductor and output filter capacitor sizes,

but 200kHz operation may be chosen for a small (about

1%) increase in efficiency at heavy loads.

Internal Reference

The internal 3.3V bandgap reference (REF) remains

active, even when the switching regulator is turned off.

It can furnish up to 5mA, and can be used to supply

memory keep-alive power or for other purposes.

Bypass REF to GND with 0.22µF, plus 1µF/mA of load

current.

Applications Information and

__________________Design Procedure

Most users will be able to work with one of the standard

application circuits; others may want to implement a

circuit with an output current rating that lies between or

beyond the standard values.

If you want an output current level that lies between two

of the standard application circuits, you can interpolate

many of the component values from the values given

for the two circuits. These components include the

input and output filter capacitors, the inductor, and the

sense resistor. The capacitors must meet ESR and rip-

ple current requirements (see

Input Filter Capacitor and

Output Filter Capacitor sections). The inductor must

meet the required current rating (see

Inductor section).

You may use the rectifier and MOSFETs specified for

the circuit with the greater output current capability, or

choose a new rectifier and MOSFETs according to the

requirements detailed in the

Rectifier and MOSFET

Switches sections. For more complete information, or

for output currents in excess of 10A, refer to the design

information in the following sections.

Inductor, L1

Three inductor parameters are required: the inductance

1.32

L1 = ______________

where:

f = switching frequency, normally 300kHz

LIR = ratio of inductor peak-to-peak AC

current to average DC load current,

typically 0.3.

A higher LIR value allows smaller inductance, but

results in higher losses and ripple.

current is typically chosen as 30% of the maximum DC

The peak inductor current at any load is given by:

1.32

2 x f x L1

The coil resistance should be as low as possible,

preferably in the low milliohms. The coil is effectively in

series with the load at all times, so the wire losses alone

are approximately:

In general, select a standard inductor that meets the L,

will fit the core.

Current-Sense Resistor, R1

The current-sense resistor must carry the peak current

in the inductor, which exceeds the full DC load current.

The internal current limiting starts when the voltage

across the sense resistors exceeds 100mV nominally,

80mV minimum. Use the minimum value to ensure ade-

start-up under full load or with load transients from no-

load to full load. If the supply is subjected to these con-

ditions, reduce the sense resistor:

70mV

R1 = ———

Since the sense-resistance values are similar to a few

centimeters of narrow traces on a printed circuit board,

trace resistance can contribute significant errors. To

prevent this, Kelvin connect the CS and FB pins to the

sense resistors; i.e., use separate traces not carrying

any of the inductor or load current, as shown in Figure 2.

5V-to-3.3V, Synchronous, Step-Down

Power-Supply Controller

______________________________________________________________________________________

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