MAX16936(2018) 데이터 시트보기 (PDF) - Maxim Integrated

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MAX16936
(Rev.:2018)

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

Maxim Integrated 

MAX16936/MAX16938

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

with 28µA Quiescent Current

Calculate RFB1 (OUT to FB resistor) with the following

equation:

= RFB1

RFB2







VOUT

VFB







−



1



where VFB = 1V (see the Electrical Characteristics table).

FPWM/Skip Modes

The MAX16936/MAX16938 offer a pin selectable skip mode

or fixed-frequency PWM mode option. The IC has an internal

LS MOSFET that turns on when the FSYNC pin is connect-

ed to VBIAS or if there is a clock present on the FSYNC pin.

This enables the fixed-frequency-forced PWM mode opera-

tion over the entire load range. This option allows the user to

maintain fixed frequency over the entire load range in appli-

cations that require tight control on EMI. Even though the

devices have an internal LS MOSFET for fixed-frequency

operation, an external Schottky diode is still required to sup-

port the entire load range. If the FSYNC pin is connected

to GND, the skip mode is enabled on the device.

In skip mode of operation, the converter’s switching

frequency 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.

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

Inductor Selection

Three key inductor parameters must be specified

for operation with the devices: inductance value (L),

inductor saturation current (ISAT), and DC resistance

(RDCR). To select inductance value, the ratio of induc-

tor 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:

ESRIN

=

∆VESR

IOUT

+

∆IL

2

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