ST777/778/779

4/11

TYPICAL APPLICATION CIRCUIT

APPLICATIONS INFORMATION

R1 and R2 must be placed only in ST779

applications to set the output voltage according to

the following equation:

and to simplify the resistor selection:

It is possible to use a wide range of values for R2

(10K

Ω to 50KΩ) with no significant loss of

accuracy thanks to the very low FB input current.

To have 1% error, the current through R2 must be

at least 100 times FB’s bias current.

When large values are used for the feedback

resistors (R1>50K

Ω), stray output impedance at

FB can incidentally add "lag" to the feedback

response, destabilizing the regulator and creating

a larger ripple at the output. Lead lengths and

circuit board traces at the FB node should be kept

short. Compensate the loop by adding a "lead"

compensation capacitor (C3, 100pF to 1nF) in

parallel with R1.

The typical value of the L1 inductor is 22µH,

enough for most applications. However, are also

suitable values ranging from 10µF to 47µF with a

saturation rating equal to or greater than the peak

switch -current limit.

Efficiency will be reduced if the inductor works

near its saturation limit, while will be maximized

using an inductor with a low DC resistance,

preferably under 0.2

Ω.

limit (1A) is set. If this maximum value is not

required is possible to reduce it connecting a

choose the right value). The current limit value is

misured when the switch current through the

inductor begins to flatten and does’nt coincide with

the max short circuit current.

Even if the device is designed to tolerate a short

circuit

without

any

damage,

it

is

strictly

recommended to avoid a continuos and durable

short circuit of the output to GND.

To achieve the best performances from switching

power supply topology, particular care to layout

drawing is needed, in order to minimize EMI and

obtain low noise. Moreover, jitter free operation

ensures the full device functionality. Wire lengths

must be minimized, filter and by-pass capacitors

must be low ESR type, placed as close as

possible to the integrated circuit. Solder AGND

and PGND pins directly to a ground plane.

ST777

ST778

ST779

ILIM

L1

22µH

SEL / N.C. / FB

R1

V+

LX

Vout

SHDN

____

AGND

C1

22µF

C2

100µF

C3

Vin

1÷6V

1

3

8

7

2

5

6

4

Vo

PGND

R2