MAX5072ETJ View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

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

ripple of 100mV or less, yielding an ESR and capaci-

tance value of 20mΩ and 6.8µF for 1.25MHz frequency.

Use a 100µF capacitor at low input voltages to avoid

possible undershoot below the undervoltage lockout

threshold during power-on and transient loading.

Output Capacitor Selection

The allowable output ripple voltage and the maximum

deviation of the output voltage during step load currents

determines the output capacitance and its ESR.

The output ripple is comprised of ∆VQ (caused by the

capacitor discharge) and ∆VESR (caused by the ESR of

the capacitor). Use low-ESR ceramic or aluminum elec-

trolytic capacitors at the output. For aluminum electrolytic

capacitors, the entire output ripple is contributed by

∆VESR. Use the ESROUT equation to calculate the ESR

requirement and choose the capacitor accordingly. If

using ceramic capacitors, assume the contribution to

the output ripple voltage from the ESR and the capacitor

discharge are equal. Calculate the output capacitance

and ESR required for a specified ripple using the follow-

ing equations:

ESROUT

=

∆VESR

∆IL

COUT = 8

×

∆IL

∆VQ ×

fSW

where

∆VO _ RIPPLE ≅ ∆VESR + ∆VQ

where ∆IL is the peak-to-peak inductor current as cal-

culated above and fSW is the individual converter’s

switching frequency.

The allowable deviation of the output voltage during

fast transient loads also determines the output capaci-

tance and its ESR. The output capacitor supplies the

step load current until the controller responds with a

greater duty cycle. The response time (tRESPONSE)

depends on the closed-loop bandwidth of the convert-

er. The high switching frequency of MAX5072 allows for

higher closed-loop bandwidth, reducing tRESPONSE

and the output capacitance requirement. The resistive

drop across the output capacitor ESR and the capaci-

tor discharge causes a voltage droop during a step

load. Use a combination of low-ESR tantalum and

ceramic capacitors for better transient load and

ripple/noise performance. Keep the maximum output

voltage deviation above the tolerable limits of the elec-

tronics being powered. When using a ceramic capaci-

tor, assume 80% and 20% contribution from the output

capacitance discharge and the ESR drop, respectively.

Use the following equations to calculate the required

ESR and capacitance value:

ESROUT

=

∆VESR

ISTEP

COUT = ISTEP

× tRESPONSE

∆VQ

where ISTEP is the load step and tRESPONSE is the

response time of the controller. Controller response

time depends on the control-loop bandwidth.

Boost Converter

The MAX5072 can be configured for step-up conversion

since the internal MOSFET can be used as a low-side

switch. Use the following equations to calculate the

inductor (LMIN), input capacitor (CIN), and output capac-

itor (COUT) when using the converter in boost operation.

Inductor

Choose the minimum inductor value so the converter

remains in continuous mode operation at minimum out-

put current (IOMIN).

LMIN

=

2

×

V2IN

fSW

×

×

D× η

VO × IOMIN

where

D = VO + VD − VIN

VO + VD − VDS

and IOMIN = 0.25 x IO

The VD is the forward voltage drop of the external Schottky

diode, D is the duty cycle, and VDS is the voltage drop

across the internal switch. Select the inductor with low DC

resistance and with a saturation current (ISAT) rating high-

er than the peak switch current limit of 4.5A and 2.2A of

converter 1 and converter 2, respectively.

Input Capacitor

The input current for the boost converter is continuous

and the RMS ripple current at the input is low. Calculate

the capacitor value and ESR of the input capacitor

using the following equations.

______________________________________________________________________________________ 19

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