MAX16907 Datasheet, PDF(12/17 Page) Maxim Integrated Products – 36V, 2.2MHz Step-Down Converter with Low Operating Current

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MAX16907

36V, 2.2MHz Step-Down Converter

with Low Operating Current

Skip Mode/Standby Mode

During light-load operation, IINDUCTOR P 185mA, 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 con-

verter skips unnecessary 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 (for VOUT = 3V to 5.5V), allowing current consump-

tion from the battery to drop to only 30FA.

Overtemperature Protection

Thermal-overload protection limits the total power dissipa-

tion in the device. When the junction temperature exceeds

+175NC (typ), an internal thermal sensor shuts down

the internal bias regulator and the step-down converter,

MAX16907

VOUT

RFB1

RFB2

Figure 1. Adjustable Output-Voltage Setting

SWITCHING FREQUENCY vs. RFOSC

3.0

2.5

2.0

1.5

1.0

0.5 VIN = 14V

ILOAD = 1.5A

RFOSC (kI)

Figure 2. Switching Frequency vs. RFOSC

allowing the IC to cool. The thermal sensor turns on the IC

again after the junction temperature 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 2.2MHz switching frequency is set with

RFOSC = 12kI. 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 switching

frequency is set by RFOSC (see the Internal Oscillator sec-

tion). The exact inductor value is not critical and can be

adjusted to make trade-offs among size, cost, efficiency,

and transient response requirements. Table 1 shows a

comparison between small and large inductor sizes.

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