MAX1586A_08 Datasheet, PDF(25/30 Page) Maxim Integrated Products – High-Efficiency, Low-IQ PMICs with Dynamic Core for PDAs and Smart Phones

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MAX1586A_08 Datasheet, PDF (25/30 Pages) Maxim Integrated Products – High-Efficiency, Low-IQ PMICs with Dynamic Core for PDAs and Smart Phones

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High-Efficiency, Low-IQ PMICs with

Dynamic Core for PDAs and Smart Phones

source and reduces switching noise in the controller.

The impedance of the input capacitor at the switching

frequency should be less than that of the input source

so high-frequency switching currents do not pass

through the input source.

The output capacitor keeps output ripple small and

ensures control-loop stability. The output capacitor

must also have low impedance at the switching fre-

quency. Ceramic, polymer, and tantalum capacitors

are suitable, with ceramic exhibiting the lowest ESR

and lowest high-frequency impedance.

Output ripple with a ceramic output capacitor is

approximately:

VRIPPLE = IL(PEAK) [1/(2Ï x fOSC x COUT)]

If the capacitor has significant ESR, the output ripple

component due to capacitor ESR is:

VRIPPLE(ESR) = IL(PEAK) x ESR

Output capacitor specifics are also discussed in the

Compensation and Stability section.

Compensation and Stability

The relevant characteristics for REG1, REG2, and

REG3 compensation are:

1) Transconductance (from FB_ to CC_), gmEA

2) Current-sense amplifier transresistance, RCS

3) Feedback regulation voltage, VFB (1.25V)

4) Step-down output voltage, VOUT, in V

5) Output load equivalent resistance, RLOAD = VOUT/

ILOAD

The key steps for step-down compensation are:

1) Set the compensation RC zero to cancel the RLOAD

COUT pole.

2) Set the loop crossover at or below approximately

1/10th the switching frequency.

For example, with VIN(MAX) = 5V, VOUT = 2.5V for

REG2, and IOUT = 800mA, then RLOAD = 3.125Î©. For

REG2, RCS = 0.75V/A and gmEA = 87ÂµS.

Choose the crossover frequency, fC â¤ fOSC/10. Choose

100kHz. Then calculate the value of the compensation

capacitor, CC:

CC = (VFB/VOUT) x (RLOAD/RCS) x (gm/(2Ï x fC))

= (1.25/2.5) x (3.125/0.75) x (87 x 10-6/(6.28

x 100,000)) = 289pF

Choose 330pF, the next highest standard value.

Now select the compensation resistor, RC, so transient-

droop requirements are met. As an example, if 3% tran-

sient droop is allowed for the desired load step, the

Table 4. Compensation Parameters

PARAMETER

Error-Amplifier

Transconductance, gmEA

Current-Sense Amp

Transresistance, RCS

REG1

87ÂµS

REG2

87ÂµS

REG3

68ÂµS

0.5V/A 0.75V/A 1.25V/A

Table 5. Typical Compensation Values

COMPONENT OR

PARAMETER

VOUT

Output Current

Inductor

Load-Step Droop

Loop Crossover Freq (fC)

COUT

REG1

3.3V

1300mA

3.3ÂµH

100kHz

330pF

240kÎ©

22ÂµF

REG2

2.5V

900mA

6.8ÂµH

100kHz

270pF

240kÎ©

22ÂµF

REG3

1.3V

500mA

10ÂµH

100kHz

330pF

240kÎ©

22ÂµF

input to the error amplifier moves 0.03 x 1.25V, or

37.5mV. The error-amplifier output drives 37.5mV x

gmEA, or IEAO = 37.5mV x 87ÂµS = 3.26ÂµA across RC to

provide transient gain. Find the value of RC that allows

the required load-step swing from:

RC = RCS x IIND(PK)/IEAO

where IIND(PK) is the peak inductor current. In a step-

down DC-DC converter, if LIDEAL is used, output cur-

rent relates to inductor current by:

IIND(PK) = 1.25 x IOUT

So for an 800mA output load step with VIN = 3.6V and

VOUT = 2.5V:

RC = RCS x IIND(PK)/IEAO = (0.75V/A) x

the response in this case since it can ramp at (VIN -

VOUT)/L, or (3.6 - 2.5)/3.3ÂµH = 242mA/Âµs.

The output filter capacitor is then selected so that the

COUT RLOAD pole cancels the RC CC zero:

COUT x RLOAD = RC x CC

For the example:

RLOAD = VOUT x ILOAD = 2.5V/0.8A =

3.125Î©

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