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LTM4602

4602fa

Equations for setting frequency for 5V to 3.3V:

ION = (VIN – 0.7V)/110k; ION = 39μA

frequency = (ION/[2.4V • 10pF]) • DC = 1.07MHz;

DC = duty cycle, duty cycle is (VOUT/VIN)

tS = tON + tOFF, tON = on-time, tOFF = off-time of the

switching period; tS = 1/frequency

tOFF must be greater than 400ns, or tS – tON > 400ns.

tON = DC • tS

~450kHz frequency or 2.22μs period is chosen for 5V to

3.3V. Frequency range is about 450kHz to 650kHz from

4.5V to 7V input.

tON = 0.66 • 2.22μs ≅ 1.46μs

tOFF = 2.22μs – 1.46μs ≅ 760ns

tON and tOFF are above the minimums with adequate guard

band.

Using the frequency = (ION/[2.4V • 10pF]) • DC, solve for

ION = (450kHz • 2.4V • 10pF) • (1/0.66) ≅ 16μA. ION current

calculated from 5V input was 39μA, so a resistor from fADJ

to ground = (0.7V/30.1k) = 23μA. 39μA – 23μA = 16μA,

sets the adequate ION current for proper frequency range

for the higher duty cycle conversion of 5V to 3.3V. Input

voltage range is limited to 4.5V to 7V. Higher input voltages

can be used without the 30.1k on fADJ. The inductor ripple

current gets too high above 7V, and the 400ns minimum

off-time is limited below 4.5V.

In 12V to 3.3V applications, if a 35k resistor is added from

the fADJ pin to ground, then a 2% efﬁciency gain will be

achieved as shown in the 12V efﬁciency graph in the Typi-

cal Performance Characteristics. This is due to the lower

transition losses in the power MOSFETs after lowering the

switching frequency down from 1.3MHz to 950kHz.

APPLICATIONS INFORMATION

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