SP6125 datasheet(4/12 Pages) SIPEX

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Mar22-07 RevC

SP6125: TSOT-6 PFET Buck Controller

 2007 Sipex Corporation

The SP6125 is a fixed frequency, Voltage-

mode,

non-synchronous

PWM

controller

optimized for minimum component, small form

factor and cost effectiveness. It has been

designed for single-supply operation ranging

from 4.5V to 29V. SP6125 has Type-II internal

compensation for use with Electrolytic or

Tantalum

output

capacitors.

For

ceramic

capacitors

Type-III

compensation

can

implemented by simply adding an R and C

between output and Feedback. A precision

0.6V reference, present on the positive terminal

of the 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 300kHz.

SP6125 contains useful protection features.

Overcurrent protection is based on high-side

MOSFET’s RDS(ON) and is programmable via a

resistor placed at LX node. Under-Voltage

Lock-Out (UVLO) ensures that the controller

starts functioning only when sufficient voltage

exists for powering IC’s internal circuitry.

SP6125 Loop Compensation

The

SP6125

includes

Type-II

internal

compensation components for loop compen-

sation. 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-II internal compensation is sufficient if the

following condition is met:

DBPOLE

ESRZERO

………………. (1)

where:

OUT

ESR

ESRZERO

……….. (2)

OUT

DBPOLE

⋅

………… (3)

⋅

……………………….. (4)

Creating a Type-III 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-III 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.

Table1- Selection of RZ

fESRZERO÷fDBPOLE RZ

50K

40K

30K

10K

>= 10X

General Overview

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