MAX16909_12 Datasheet, PDF(12/18 Page) Maxim Integrated Products – 36V, 220kHz to 1MHz Step-Down Converter with Low Operating Current

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MAX16909_12 Datasheet, PDF (12/18 Pages) Maxim Integrated Products – 36V, 220kHz to 1MHz Step-Down Converter with Low Operating Current

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MAX16909

36V, 220kHz to 1MHz Step-Down Converter

with Low Operating Current

Skip Mode/Standby Mode

During light-load operation the device enters skip mode

operation. Skip mode turns off the majority of circuitry

and allows the output to drop below regulation voltage

before the switch is turned on again. The lower the load

current, the longer it takes for the regulator to initiate

a new cycle. Because the converter skips unneces-

sary cycles and turns off the majority of circuitry, the

converter efficiency increases. When the high-side FET

stops switching for more than 50Fs, most of the internal

circuitry, including LDO, draws power from VOUT (VOUT

= 3V to 5.5V), allowing current consumption from the bat-

tery to drop to only 30FA.

Overtemperature Protection

Thermal-overload protection limits the total power dis-

sipation in the device. When the junction temperature

exceeds +175NC (typ), an internal thermal sensor

shuts down the internal bias regulator and the step-

MAX16909

VOUT

RFB1

RFB2

Figure 1. Adjustable Output-Voltage Setting

SWITCHING FREQUENCY vs. RFOSC

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1 VIN = 14V

20 30 40 50 60 70 80 90 100 110

RFOSC (kI)

Figure 2. Switching Frequency vs. RFOSC

Maxim Integrated

down converter, allowing the IC to cool. The thermal

sensor turns on the IC again after the junction tempera-

ture cools by 15NC.

Applications Information

Setting the Output Voltage

Connect FB to BIAS for a fixed 5V output voltage. To set

the output to other voltages between 1V and 10V, con-

nect a resistive divider from output (OUT) to FB to GND

(Figure 1). Calculate RFB1 (OUT to FB resistor) with the

following equation:

= RFB1

RFB2

ï£®ï£«

ï£¯ï£¬

ï£¯ï£°ï£

VOUT

VFB

ï£¶

ï£·

ï£¸

ï£¹

1ï£º

ï£ºï£»

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

Internal Oscillator

The switching frequency, fSW, is set by a resistor (RFOSC)

connected from FOSC to GND. See Figure 2 to select the

correct RFOSC value for the desired switching frequency.

For example, a 400kHz switching frequency is set with

RFOSC = 65kI. Higher frequencies allow designs with

lower inductor values and less output capacitance.

Consequently, peak currents and I2R losses are lower

at higher switching frequencies, but core losses, gate

charge currents, and switching losses increase.

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 â VOUT )

VSUPfSWIOUTLIR

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

efficiency is optimum for typical conditions). The switch-

ing frequency is set by RFOSC (see the Internal Oscillator

section). The exact inductor value is not critical and can

be adjusted to make trade-offs among size, cost, efficien-

cy, and transient response requirements. Table 1 shows

a comparison between small and large inductor sizes.

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