SP6120 datasheet(8/22 Pages) SIPEX

8 / 22 page

Date: 1/21/05

SP6120 Low Voltage, AnyFETTM, Synchronous, Buck Controller

General Overview

The SP6120 is a constant frequency, voltage

mode PWM controller for low voltage, DC/DC

step down converters. It has a main loop where

an external resistor (ROSC) sets the frequency

and the driver is controlled by the comparison of

an error amp output (COMP) and a 1V ramp

signal. The error amp has a transconductance of

600

µS, an output impedance of 3 MΩ, an inter-

nal pole at 2 MHz and a 1.25V reference input.

Although the main control loop is capable of 0%

and 100% duty cycle, its response time is lim-

ited by the external component selection. There-

fore, a secondary loop, including a window

comparator positioned 3% above and below the

reference, has been added to insure fast re-

sponse to line and load transients. A unique

“Ripple & Frequency Independent” algorithm,

added to the secondary loop, insures that the

window comparator does not interfere with the

main loop during normal operation. In addition

to receiving driver commands from the main

and secondary loops, the Driver Logic is also

controlled by the Programming Logic, Fault

Logic and Zero Crossing Comparator. The Pro-

gramming Logic tells the Driver Logic whether

the controller is using a PFET or NFET high side

Figure 14. V

CC Start Threshold vs. Temperature

Figure 15. V

CC Stop Threshold vs. Temperature

Figure 16. Oscillator Frequency vs. Temperature

Figure 17. Oscillator Frequency vs. R

OSC with VCC = 5V

and CGH = CGL = Open.

2.900

-40

-15

Temperature (

°C)

2.875

2.850

2.825

2.800

2.850

-40

-15

Temperature (

°C)

2.825

2.800

2.775

2.750

302

-40

-15

Temperature (

°C)

301

300

299

298

200

300

400

500

600

700

800

510

2025

ROSC (kΩ)

TYPICAL PERFORMANCE CHARACTERISTICS: Continued

Unless otherwise specified: VCC = BST = ENABLE = 3.3V, ROSC = 18.7k, CCOMP = 0.1µF, CSS = 0.1µF, CGH = CGL = 3.3nF, VFB = 1.25V,

ISP = ISN = 1.25V, SWN = GND = PGND = 0V, TAMB = 25°C.

THEORY OF OPERATIONS

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