SP6121CNSLASHTR datasheet(12/16 Pages) SIPEX

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Date: 5/25/04

SP6121 Low Voltage, Synchronous Step Down PWM Controller

associated with the output capacitors and can be

determined by

fZ(ESR) =

πCOUTRESR

Crossover frequency of 20kHz is a sound first

try if low ESR tantalum capacitors or poscaps

are used at the output. The next step is to calcu-

late the complex conjugate poles contributed by

the LC output filter,

fP(LC) =

π√ LCOUT

The open loop gain of the whole system can be

divided into the gain of the error amplifier,

PWM modulator, buck converter, and feedback

resistor divider. In order to crossover at the

selected frequency fco, the gain of the error

amplifier has to compensate for the attenuation

caused by the rest of the loop at this frequency.

In the RC network shown in Figure 4, the product

of R1 and the error amplifier transconductance

determines this gain. Therefore, R1 can be deter-

mined from the following equation that takes

into account the typical error amplifier

transconductance, reference voltage and PWM

ramp built into the SP6121.

R1 =

975VOUT fCO fZ(ESR)

VIN fP(LC)2

In Figure 4, R1 and C1 provides a zero fZ1 which

needs to be placed at or below fP(LC). If fZ1 is

made equal to fP(LC) for convenience, the value

of C1 can be calculated as

C1 =

πfP(LC)R1

The optional C2 generates a pole fP1 with R1 to

cut down high frequency noise for reliable op-

eration. This pole should be placed one decade

higher than the crossover frequency to avoid

erosion of phase margin. Therefore, the value of

the C2 can be derived from

C2 =

πfCOR1

Figure 5 illustrates the overall loop frequency

response and frequency of each pole and zero.

To fine-tune the compensation, it is necessary to

physically measure the frequency response us-

ing a network analyzer.

Gain

-20db/dec

-40db/dec

-20db/dec

Error Amplifier

Loop

Figure 5. Frequency response of a stable system and its

error amplifier.

Overcurrent Protection

Over current protection on the SP6121 is imple-

mented through detection of an excess voltage

condition across the high side PMOS switch

during conduction. This is typically referred to

as high side RDS(ON) detection. By using the

RDS(ON) of Q1 to measure the output current, the

current limit circuit eliminates the sense resistor

that would otherwise be required and the corre-

sponding loss associated with it. This improves

the overall efficiency and reduces the number of

components in the power path benefiting size

and cost. RDS(ON) sensing is by default inaccu-

rate and is primarily meant to protect the power

supply during a fault condition. The overcurrent

trip point will vary from unit to unit as the

RDS(ON) of Q1 varies. The SP6121 provides a

built-in 160mV threshold between the ISET and

ISENSE pins. If a current limit threshold in excess

of 160mV is required, an external programming

resistor, RSET can be added between ISET pin and

VIN as shown in Figure 6.

APPLICATIONS INFORMATION: Continued

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