MAX16936 データシートの表示（PDF） - Maxim Integrated

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MAX16936

36V, 220kHz to 2.2MHz Step-Down Converter with 28µA Quiescent Current

Maxim Integrated 

MAX16936

36V, 220kHz to 2.2MHz Step-Down Converter

with 28µA Quiescent Current

In skip mode of operation, the converter’s switching fre-

quency is load dependent. At higher load current, the

switching frequency does not change and the operating

mode is similar to the FPWM mode. Skip mode helps

improve efficiency in light-load applications by allowing

the converters to turn on the high-side switch only when

the output voltage falls below a set threshold. As such,

the converters do not switch MOSFETs on and off as

often as is the case in the FPWM mode. Consequently,

the gate charge and switching losses are much lower in

skip mode.

Inductor Selection

Three key inductor parameters must be specified for

operation with the device: inductance value (L), inductor

saturation current (ISAT), and DC resistance (RDCR). To

select inductance value, the ratio of inductor peak-to-

peak AC current to DC average current (LIR) must be

selected first. A good compromise between size and loss

is a 30% peak-to-peak ripple current to average current

ratio (LIR = 0.3). The switching frequency, input voltage,

output voltage, and selected LIR then determine the

inductor value as follows:

L = VOUT (VSUP − VOUT)

VSUP fSW IOUT LIR

where VSUP, VOUT, and IOUT are typical values (so that

efficiency is optimum for typical conditions). The switching

frequency is set by RFOSC (see Figure 3).

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 capacitor RMS current requirement (IRMS) is

defined by the following equation:

IRMS = ILOAD(MAX)

VOUT (VSUP − VOUT)

VSUP

IRMS has a maximum value when the input voltage

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

IRMS(MAX) = ILOAD(MAX)/2.

Choose an input capacitor that exhibits less than +10NC

self-heating temperature rise at the RMS input current for

optimal long-term reliability.

The input voltage ripple is composed of DVQ (caused

by the capacitor discharge) and DVESR (caused by the

ESR of the capacitor). Use low-ESR ceramic capacitors

with high ripple current capability at the input. Assume

the contribution from the ESR and capacitor discharge

equal to 50%. Calculate the input capacitance and ESR

required for a specified input voltage ripple using the fol-

lowing equations:

where:

ESRIN

=

∆VESR

IOUT

+

∆IL

2

SWITCHING FREQUENCY vs. RFOSC

2.50

2.25

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

0

12

42

72

102

132

RFOSC (kΩ)

Figure 3. Switching Frequency vs. RFOSC

and:

∆IL

=( VSUP − VOUT ) × VOUT

VSUP × fSW × L

= CIN

IO= UT × D(1− D) and D

∆VQ × fSW

VOUT

VSUPSW

where IOUT is the maximum output current and D is the

duty cycle.

Output Capacitor

The output filter capacitor must have low enough ESR

to meet output ripple and load transient requirements.

The output capacitance must be high enough to absorb

the inductor energy while transitioning from full-load

to no-load conditions without tripping the overvoltage

fault protection. When using high-capacitance, low-ESR

capacitors, the filter capacitor’s ESR dominates the output

Maxim Integrated

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