SP7656EN2-LSLASHTR datasheet(4/10 Pages) EXAR

4 / 10 page

11/07/08 SP7656 PowerBlox



SP7656 General Overview:

The SP7656 PowerBlox is a fixed frequency,

voltage

mode,

non-synchronous

PWM

controller co-packaged with a P-Channel FET.

SP7656 is optimized for minimum component,

small form factor and cost effectiveness. It has

been designed for single-supply operation

ranging from 4.5V to 29V. SP7656 has Type-2

internal

compensation

for

use

with

Electrolytic/Tantalum output capacitors. For

ceramic capacitors Type-3 compensation can

be implemented by simply adding an R and C

between output and Feedback.

A precision 0.6V reference, present on the

positive terminal of the internal error amplifier,

permits programming of the output voltage

down to 0.6V via the FB pin. The output of the

Error Amplifier is internally compared to a

feed-forward (VIN/5 peak-to-peak) ramp and

generates the PWM control.

Timing is governed by an internal oscillator

that sets the PWM frequency at 600kHz.

SP7656 contains useful protection features.

Over-current protection is based on the

internal

MOSFET

Rds(on)

and

programmable via a resistor placed between

ISET and LX node. Under-Voltage Lock-Out

(UVLO) ensures that the controller starts

functioning only when sufficient voltage exists

for powering IC’s internal circuitry.

SP7656 Loop Compensation

The

SP7656

includes

Type-2

internal

compensation

components

for

loop

compensation.

External

compensation

components are not required for systems with

tantalum or aluminum electrolytic output

capacitors with sufficiently high ESR. Use the

condition below as a guideline to determine

whether or not the internal compensation is

sufficient for your design. Type-2 internal

compensation is sufficient if the following

condition is met:

DBPOLE

ESRZERO



where:

OUT

ESR

ESRZERO





OUT

DBPOLE





Creating a Type-3 compensation Network

The above condition requires the ESR zero to

be at a lower frequency than the double-pole

from the LC filter. If this condition is not met,

Type-3 compensation should be used and can

be accomplished by placing a series RC

combination in parallel with R1 as shown

below. The value of CZ can be calculated as

follows and RZ selected from table 1.

CZ 

Table1- RZ Selection

200k, 1%

SP7656

RZ2

200k

CZ2

130pF

CP1

2pF

Error Amplif ier

Vref =0.6V

Vout

Figure 1- RZ and CZ in conjunction with internal

compensation components form a Type-3

compensation

fESRZERO/fDBPOLE RZ

50K

40K

30K

10K

>= 10X

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