MAX8655ETN(2007) 데이터 시트보기 (PDF) - Maxim Integrated

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MAX8655ETN
(Rev.:2007)

Highly Integrated, 25A, Wide-Input, Internal MOSFET, Step-Down Regulator

Maxim Integrated 

Highly Integrated, 25A, Wide-Input,

Internal MOSFET, Step-Down Regulator

Capacitor C11 is connected in parallel with R2 and is

equal in value with C9.

Add a 100pF (C10) capacitor across the CS+ and CS-

inputs close to the IC.

Input Capacitor

The input filter capacitor reduces peak currents drawn

from the power source and reduces noise and voltage

ripple on the input caused by the circuit’s switching.

The input capacitors must meet the ripple-current

requirement (IRMS) imposed by the switching currents

defined by the following equation:

( ) IRMS = ILOAD

VOUT × VIN − VOUT

VIN

IRMS has a maximum value when the input voltage

equals twice the output voltage (VIN = 2 x VOUT), so

IRMS(MAX) = ILOAD / 2. Ceramic capacitors are recom-

mended due to the low ESR and ESL at high frequency

with relatively low cost. Choose a capacitor that

exhibits less than 10°C temperature rise at the maxi-

mum operating RMS current for optimum long-term reli-

ability. Ceramic capacitors with an X5R or better

temperature characteristic are recommended.

Output Capacitor

The key selection parameters for the output capacitor

are the actual capacitance value, the equivalent series

resistance (ESR), the equivalent series inductance

(ESL), and the voltage-rating requirements. These

parameters affect the overall stability, output-voltage

ripple, and transient response. The output ripple has

three components: variations in the charge stored in

the output capacitor, the voltage drop across the

capacitor’s ESR, and ESL caused by the current into

and out of the capacitor. The maximum output-voltage

ripple is estimated as follows:

VRIPPLE = VRIPPLE(ESR) + VRIPPLE(C) + VRIPPLE(ESL)

The output-voltage ripple as a consequence of the

ESR, ESL, and output capacitance is:

VRIPPLE(ESR) = IP−P × ESR

VRIPPLE(ESL)

=

L

VIN

+ ESL

×

ESL

VRIPPLE(C)

=

8

×

IP−P

COUT

×

fS

where IP-P is the peak-to-peak inductor current.

IP−P

=

VIN − VOUT

fS × L

×

VOUT

VIN

These equations are suitable for initial capacitor selec-

tion, but final values should be chosen based on a pro-

totype or evaluation circuit. As a general rule, a smaller

current ripple results in less output-voltage ripple.

Since the inductor ripple current is a factor of the

inductor value and input voltage, the output-voltage rip-

ple decreases with larger inductance, and increases

with higher input voltages. The MAX8655 is designed to

work with polymer, tantalum, aluminum electrolytic, or

ceramic output capacitors. The aluminum electrolytic

capacitor is the least expensive; however, it has higher

ESR. To compensate for this, use a ceramic capacitor

in parallel to reduce the switching ripple and noise.

Ceramic capacitors are recommended for high-fre-

quency (500kHz to 1MHz) designs. For reliable and

safe operation, ensure that the capacitor’s voltage and

ripple-current ratings exceed the calculated values.

The response to a load transient depends on the

selected output capacitors. During a load transient, the

output voltage instantly changes by ESR x ΔILOAD.

Before the regulator can respond, the output voltage

deviates further, depending on the inductor and output-

capacitor values. After a short time (see the Typical

Operating Characteristics section), the regulator

responds by regulating the output voltage back to its

nominal state. The regulator response time depends on

its closed-loop bandwidth. With a higher bandwidth,

the response time is faster, thus preventing the output

voltage from further deviation from its regulating value.

Compensation Design

The MAX8655 uses an internal transconductance error

amplifier whose output compensates the control loop.

The external inductor, output capacitor, compensation

resistor, and compensation capacitors determine the

loop stability. The inductor and output capacitor are cho-

sen based on performance, size, and cost. Additionally,

the compensation resistor and capacitors are selected to

optimize control-loop stability. The component values,

shown in Figures 3 and 4, yield stable operation over the

given range of input-to-output voltages.

The regulator uses a current-mode control scheme that

regulates the output voltage by forcing the required cur-

rent through the external inductor. The voltage drop

across the DC resistance of the inductor or the alternate

series current-sense resistor is used to measure the

inductor current. Current-mode control eliminates the

double pole in the feedback loop caused by the

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