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The inductance (H) value is then given by:

Diode Selection

The output diode should be rated to handle the output

voltage and the peak switch current. Make sure the

diode’s peak current rating is at least IPK and that its

breakdown voltage exceeds VOUT. Schottky diodes are

recommended. If a junction rectifier is used, it must be

an ultra-fast type (trr < 50ns) to prevent excessive loss

in the rectifier.

Input and Output Capacitor Selection

The MAX1896 operates with both tantalum and ceramic

output capacitors. When using tantalum capacitors, the

zero caused by the ESR of the tantalum is used to

ensure stability. When using ceramic capacitors, the

zero due to the ESR will be at too high a frequency to

be useful in stabilizing the control loop. When using

ceramic capacitors, use a feedforward capacitor to

increase the phase margin, improving the control-loop

stability. Figure 3 shows the circuit with ceramic capac-

itors and the feedforward capacitor, CFF. Use the fol-

lowing equation to determine the value of the

feedforward capacitor:

where:

R1 = see Figure 3, (Ω)

COUT = total output capacitance including any bypass

capacitor on the output bus, (Farads). See Figure 3.

VOUT = output voltage, (V)

VIN = input voltage, (V).

Setting the Output Voltage

The MAX1896 operates with an adjustable output from

VIN to 13V. Connect a resistive voltage-divider from the

output to FB (see Typical Operating Circuit). Choose a

value for R2 between 10kΩ and 50kΩ. Calculate R1

using the equation:

where VFB, the step-up regulator feedback set point, is

1.24V. Connect the resistive-divider as close to the IC

as possible.

Soft-Start Capacitor

The soft-start capacitor should be large enough that the

current limit does not reach final value before the output

has reached regulation. Calculate CSS to be:

where:

k2 = 21 x 10-6, (S)

VOUT = maximum output voltage, (V)

IINRUSH = peak inrush current allowed, (A)

IOUT = maximum output current during power-up stage, (A)

VIN = minimum input voltage, (V)

The soft-start duration (tSS) is the time it takes the cur-

rent limit to reach its final value. The soft-start duration

can be calculated by the equation:

tss = k3 ✕ CSS

where:

k3 = 6.67 ✕ 105Ω

x C

x V

Vx I

x V

OUT

INRUSH

OUT













−

OUT













−

with units of

17 14













−

Ω

Cx V

OUT













x V

Vx LIR x I

x f

IN MIN

OUT

IN MIN

OUT

OUT MAX

OSC

[

(

)]

()

−

1.4MHz SOT23 Current-Mode

Step-Up DC-DC Converter

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