SP6127 datasheet(4/12 Pages) SIPEX

4 / 12 page

June 26, 2007

SP6127 TSOT-6 PFET Buck Controller

 2007 Sipex Corporation

Page 4

____________________________________________________ GENERAL OVERVIEW

The SP6127 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.

SP6127 has Type-II internal compensation for

use with Electrolytic/Tantalum output capacitors.

For ceramic capacitors Type-III 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 Error Amplifier, permits programming of the

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

output

the

Error

Amplifier

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 900kHz.

SP6127 contains useful

protection

features.

Overcurrent protection is based on the 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.

SP6127 Loop Compensation

The SP6127 includes Type-II 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-II internal compensation is sufficient if the

following condition is met:

DBPOLE

ESRZERO

………………. (1)

where:

OUT

ESR

ESRZERO

……….. (2)

OUT

DBPOLE

⋅

………… (3)

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

placing

series

combination in parallel with R1 as shown below.

The value of CZ can be calculated as follows and

RZ selected from table 1.

⋅

………….. (4)

Table1- Selection of

fESRZERO/

fDBPOLE

50K

40K

30K

10K

>= 10X

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