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MAX5072ETJ

2.2MHz, Dual-Output Buck or Boost Converter with POR and Power-Fail Output

Maxim Integrated 

2.2MHz, Dual-Output Buck or Boost

Converter with POR and Power-Fail Output

For output voltages below 0.8V, set the MAX5072 out-

put voltage by connecting a voltage-divider from the

output to FB_ to BYPASS (Figure 6). Select RC (FB to

BYPASS resistor) higher than a 50kΩ range. Calculate

RA with the following equation:

RA =

RC

⎡

⎢

⎣

VFB − VOUT

VBYPASS − VFB

⎤

⎥

⎦

where VFB = 0.8V, VBYPASS = 2V (see the Electrical

Characteristics table), and VOUT_ can range from 0V to

VFB_.

Inductor Selection

Three key inductor parameters must be specified for

operation with the MAX5072: inductance value (L), peak

inductor current (IL), and inductor saturation current

(ISAT). The minimum required inductance is a function of

operating frequency, input-to-output voltage differential

and the peak-to-peak inductor current (∆IL). Higher ∆IL

allows for a lower inductor value while a lower ∆IL

requires a higher inductor value. A lower inductor value

minimizes size and cost, improves large-signal transient

response, but reduces efficiency due to higher peak cur-

rents and higher peak-to-peak output ripple voltage for

the same output capacitor. On the other hand, higher

inductance increases efficiency by reducing the ripple

current. However, resistive losses due to extra wire turns

can exceed the benefit gained from lower ripple current

levels, especially when the inductance is increased with-

out also allowing for larger inductor dimensions. A good

compromise is to choose ∆IL equal to 30% of the full

load current. To calculate the inductance use the follow-

ing equation:

( ) L = VOUT VIN − VOUT

VIN × fSW × ∆IL

where VIN and VOUT are typical values (so that efficiency

is optimum for typical conditions). The switching frequen-

cy is set by ROSC (see the Setting the Switching

Frequency section). The peak-to-peak inductor current,

which reflects the peak-to-peak output ripple, is worst at

the maximum input voltage. See the Output Capacitor

Selection section to verify that the worst-case output rip-

ple is acceptable. The inductor saturating current is also

important to avoid runaway current during the output

overload and continuous short circuit. Select the ISAT to

be higher than the maximum peak current limits of 4.5A

and 2.2A for converter 1 and converter 2.

Input Capacitors

The discontinuous input current waveform of the buck

converter causes large ripple currents at the input. The

switching frequency, peak inductor current, and the

allowable peak-to-peak voltage ripple dictate the input

capacitance requirement. Increasing the switching fre-

quency or the inductor value lowers the peak to aver-

age current ratio, yielding a lower input capacitance

requirement. Note that two converters of MAX5072 run

180° out-of-phase, thereby effectively doubling the

switching frequency at the input.

The input ripple waveform would be unsymmetrical due

to the difference in load current and duty cycle between

converter 1 and converter 2. The input ripple is com-

prised of ∆VQ (caused by the capacitor discharge) and

∆VESR (caused by the ESR of the capacitor). A higher

load converter dictates the ESR requirement, while the

capacitance requirement is a function of the loading

mismatch between the two converters. The worst-case

mismatch is when one converter is at full load while the

other is at no load or in shutdown. Use low-ESR ceramic

capacitors with high ripple-current capability at the

input. Assume the contribution from the ESR and capac-

itor discharge equal to 50%. Calculate the input capaci-

tance and ESR required for a specified ripple using the

following equations:

ESRIN

=

∆VESR

⎛⎝⎜IOUT

+

∆IL

2

⎞

⎠⎟

where

( ) ∆IL =

VIN − VOUT × VOUT

VIN × fSW × L

and

where

( ) CIN

=

IOUT ×

∆VQ

D 1− D

× fSW

D = VOUT

VIN

where IOUT is the maximum output current from either

converter 1 or converter 2, and D is the duty cycle for

that converter. fSW is the frequency of each individual

converter. For example, at VIN = 12V, VOUT = 3.3V at

IOUT = 2A, and with L = 3.3µH, the ESR and input

capacitance are calculated for a peak-to-peak input

18 ______________________________________________________________________________________

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