SP6126 datasheet(4/14 Pages) SIPEX

4 / 14 page

Mar 29-07 RevD

SP6126: TSOT-6 PFET Buck Controller

 2007 Sipex Corporation

The SP6126 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. SP6126 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 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.

SP6126 contains useful protection features.

Over-current 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.

SP6126 Loop Compensation

The

SP6126

includes

Type-II

internal

compensation

components

for

loop

compensation.

External

compensation

components are not required for systems with

tantalum

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 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.

⋅

………….. (4)

Table1- Selection of RZ

Figure 1- RZ and CZ in conjunction with

internal compensation components form

a Type-III compensation

fESRZERO ÷ fDBPOLE

50K

40K

30K

10K

>= 10X

200k, 1%

SP6126

VFB

RZ2

200k

CZ2

130pF

CP1

2pF

Error Amplif ier

Vref =0.6V

Vout

GENERAL OVERVIEW

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