MAX726 데이터 시트보기 (PDF) - Maxim Integrated

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MAX726

5A/2A Step-Down, PWM, Switch-Mode DC-DC Regulators

Maxim Integrated 

5A/2A Step-Down, PWM,

Switch-Mode DC-DC Regulators

Inductor Selection

Although most MAX724 designs perform satisfactorily

with 50µH inductors (100µH for the MAX726), the

MAX724/MAX726 are able to operate with values rang-

ing from 5µH to 200µH. In some cases, inductors other

than 50µH may be desired to minimize size (lower

inductance), or reduce ripple (higher inductance). In

any case, inductor current must at least be rated for the

desired output current.

In high-current applications, pay particular attention to

both the RMS and peak inductor ratings. The induc-

tor's peak current is limited by core saturation.

Exceeding the saturation limit actually reduces the

coil's inductance and energy storage ability, and

increases power loss. Inductor RMS current ratings

depend on heating effects in the coil windings.

The following equation calculates maximum output cur-

rent as a function of inductance and input conditions:

VOUT (VIN - VOUT)

IOUT = ISW - 2 fOSC VINL

where ISW is the maximum switch current (5.5A for

MAX724), VIN is the maximum input voltage, VOUT is the

output voltage, and fOSC is the switching frequency.

For the MAX724 example in Figure 2, with L = 50µH

and VIN = 25V,

5V (25V - 5V)

IOUT = 5.5A - 2 (105Hz) 25V (50 x 10-6H) = 5.1A

Note that increasing or decreasing inductor value pro-

vides only small changes in maximum output current

(100µH = 5.3A, 20µH = 4.5A). The equation shows that

output current is mostly a function of the

MAX724/MAX726 current-limit value. Again, a 50µH

inductor works well in most applications and provides

5A with a wide range of input voltages.

Catch Diode

D1 provides a path for inductor current when VSW turns

off. Under normal load conditions, the average diode

current may only be a fraction of load current; but dur-

ing short-circuit or current-limit, diode current is higher.

Conservative design dictates that the diode average

current rating be 2 times the desired output current. If

operation with extended short-circuit or overload time is

expected, then the diode current rating must exceed

the current limit (6.5A = MAX724, 2.6A = MAX726), and

heat sinking may be necessary.

Under normal operating conditions (not shorted), power

dissipated in the diode PD is calculated by:

PD

=

IOUT

(VIN

-

VOUT)

VIN

VD

where VD is forward drop of the diode at a current

equal to IOUT. In nearly all circuits, Schottky diodes

provide the best performance and are recommended

due to their fast switching times and low forward voltage

drop. Standard power rectifiers such as the 1N4000

series are too slow for DC-DC conversion circuits and are

not recommended.

Output Filter Capacitor

For most MAX724/MAX726 applications, a high-quality,

low-ESR, 470µF or 500µF output filter capacitor will suf-

fice. To reduce ripple, minimize capacitor lead length

and connect the capacitor directly to the GND pin.

Capacitor suppliers are listed in Table 1. Output ripple

is a function of inductor value and output capacitor

effective series resistance (ESR). In continuous-con-

duction mode:

ESR (VOUT) (1 - VOUT/VIN)

VCR(p-p) =

L fOSC

It is interesting to note that input voltage (VIN), and not

load current, affects output ripple in CCM. This is

because only the DC, and not the peak-to-peak, induc-

tor current changes with load (see Figure 3).

In discontinuous-conduction mode, the equation is dif-

ferent because the peak-to-peak inductor current does

depend on load:

√ VDR(p-p) = ESR

2 IOUT VOUT (VIN - VOUT)

L fOSC VIN

where output ripple is proportional to the square root of

load current. Refer to the earlier equation for IDCM to

determine where DCM occurs and hence when the

DCM ripple equation should be used.

Input Bypass Capacitor

An input capacitor (200µF or 220µF) is required for step-

down converters because the input current, rather than

being continuous (like output current), is a square wave.

For this reason the capacitor must have low ESR and a

ripple-current rating sufficiently large so that its ESR and

the AC input current do not conspire to overheat the

capacitor. In CCM, the capacitor's RMS ripple current is:

√ IR(RMS) = IOUT

VOUT (VIN - VOUT)

VIN2

The power dissipated in the input capacitor is then PC:

PC = IR(RMS)2 (ESR)

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