MAX15038 查看數據表（PDF） - Maxim Integrated

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MAX15038

4A, 2MHz Step-Down Regulator with Integrated Switches

Maxim Integrated 

4A, 2MHz Step-Down Regulator

with Integrated Switches

where the output ripple due to output capacitance,

ESR, and ESL is:

VRIPPLE(C)

=

8

x

IP−P

COUT

x

fS

VRIPPLE(ESR) = IP−P x ESR

VRIPPLE(ESL)

=

IP−P

tON

x ESL

or:

VRIPPLE(ESL)

=

IP−P

tOFF

x

ESL

or whichever is larger.

The peak-to-peak inductor current (IP-P) is:

IP−P

=

VIN − VOUT

fS × L

x

VOUT

VIN

Use these equations for initial output capacitor selec-

tion. Determine final values by testing a prototype or an

evaluation circuit. A smaller ripple current results in less

output-voltage ripple. Since the inductor ripple current

is a factor of the inductor value, the output-voltage rip-

ple decreases with larger inductance. Use ceramic

capacitors for low ESR and low ESL at the switching

frequency of the converter. The ripple voltage due to

ESL is negligible when using ceramic capacitors.

Load-transient response depends on the selected out-

put capacitance. During a load transient, the output

instantly changes by ESR x ΔILOAD. Before the con-

troller can respond, the output deviates further,

depending on the inductor and output capacitor val-

ues. After a short time, the controller responds by regu-

lating the output voltage back to its predetermined

value. The controller response time depends on the

closed-loop bandwidth. A higher bandwidth yields a

faster response time, preventing the output from deviat-

ing further from its regulating value. See the Compen-

sation Design section for more details.

Input-Capacitor Selection

The input capacitor reduces the current peaks drawn

from the input power supply and reduces switching

noise in the IC. The total input capacitance must be

equal or greater than the value given by the following

equation to keep the input-ripple voltage within

specification and minimize the high-frequency ripple

current being fed back to the input source:

CIN _ MIN

=

D x TS x IOUT

VIN−RIPPLE

where VIN-RIPPLE is the maximum allowed input ripple

voltage across the input capacitors and is recommended

to be less than 2% of the minimum input voltage. D is

the duty cycle (VOUT/VIN) and TS is the switching peri-

od (1/fS).

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, but are instead shunted through the

input capacitor. The input capacitor must meet the ripple

current requirement imposed by the switching currents.

The RMS input ripple current is given by:

IRIPPLE = ILOAD ×

VOUT × (VIN − VOUT )

VIN

where IRIPPLE is the input RMS ripple current.

Compensation Design

The power transfer function consists of one double pole

and one zero. The double pole is introduced by the

inductor L and the output capacitor CO. The ESR of the

output capacitor determines the zero. The double pole

and zero frequencies are given as follows:

fP1_LC = fP2 _LC =

1

2π x

L

x

CO

x

⎛

⎝⎜

RO + ESR

RO + RL

⎞

⎠⎟

fZ _ESR

=

2π

1

x ESR

x

CO

where RL is equal to the sum of the output inductor’s DCR

(DC resistance) and the internal switch resistance,

RDS(ON). A typical value for RDS(ON) is 24mΩ (low-side

MOSFET) and 31mΩ (high-side MOSFET). RO is the out-

put load resistance, which is equal to the rated output

voltage divided by the rated output current. ESR is the

total equivalent series resistance of the output capacitor.

If there is more than one output capacitor of the same

type in parallel, the value of the ESR in the above equa-

tion is equal to that of the ESR of a single output capacitor

divided by the total number of output capacitors.

The high switching frequency range of the MAX15038

allows the use of ceramic output capacitors. Since the

ESR of ceramic capacitors is typically very low, the fre-

quency of the associated transfer function zero is higher

than the unity-gain crossover frequency, fC, and the zero

cannot be used to compensate for the double pole creat-

ed by the output filtering inductor and capacitor. The dou-

ble pole produces a gain drop of 40dB/decade and a

phase shift of 180°. The compensation network error

14 ______________________________________________________________________________________

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