IRU3034CS datasheet(9/12 Pages) IRF

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RB = Rt / X

X =

[(Vo + (∆Vo/2)) / VREF] - 1

[(3.38+ (0.044/2)) / 1.25] - 1 = 1.72

RB = 1000 / 1.72 = 580Ω

Ppk = ICL2×Rs = 202×0.005 = 2W

PAVG = 0.5×Ppk×Dsc

THCP = 0.1×200 = 20ms

PAVG = 0.5×2×(0.5/20) = 25mW

RH = (11×Rt) / ∆Vo

RH = (11×1000) / 0.044 = 250KΩ

The bottom resistor of the divider is then calculated us-

ing the following equations:

Where:

RB = Bottom resistor of the divider

VREF = 1.25V Typical

Select RB = 576Ω, 1%

Frequency Calculation

The IRU3034 frequency of operation is calculated using

the following formula:

Where:

Vo = Output voltage (V)

D = Duty cycle

ESR = Output capacitor ESR (V)

L = Output inductance (µH)

∆Vo = Output ripple voltage (V)

For our example:

Where:

Vf = Forward voltage drop of the Schottky diode.

The ESR=18mΩ for 2 of the Sanyo 1500µF,

6MV1500GX caps. If L=3.5µH then, Fs is calculated as

follows:

The peak power dissipated in the CS resistor is:

However, the average power dissipated is much lower

than 2W due to the long off time caused by the hiccup

circuit of IRU3034. The average power is in fact the short

circuit period divided by the short circuit period plus the

off time or "Hiccup" period. For example, if the short

circuit lasts for Tsc=100µs before the IRU3034 enters

hiccup, the average power is calculated as:

Where:

Dsc = Tsc / THCP

THCP = Css×M

M = 200ms/µF

Css = The soft-start capacitor

For example:

For Css=0.1µF and Tsc=500µs=0.5ms

Without "Hiccup" technique, the power dissipation of the

resistor is 2W.

Switcher Output Voltage Setting

The output voltage can be set using the following

equations:

Assuming, Vo=3.38V and the selected output ripple is

≈

1.3%(44mV) of the output voltage, a set of equations

are derived that selects the resistor divider and the hys-

teresis resistor:

Assuming, Rt = 1KΩ, 1%:

Where:

Rt = Top resistor of the resistor divider

RH = Hysteresis resistor connected between pins 3

and 4 of the IRU3034

∆Vo = Selected output ripple (typically 1% to 2% of

output voltage)

Assuming, ∆Vo=44mV:

Select RH = 249KΩ, 1%

Fs =

= 0.087

Fs = 87KHz

[(3.38 × (1-0.78) × 0.018)]

(3.5 × 0.044)

D = (3.38 + 0.5) / 5 = 0.78

≈ (Vo + Vf) / VIN

Fs =

[(Vo×(1-D)×ESR)] / (L×∆Vo)

(MHz)

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