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LTM4600

4600fc

The LTM4600 has a minimum (tON)ontimeof100nanosec-

onds and a minimum (tOFF) off time of 400 nanoseconds.

The 2.4V clamp on the ramp threshold as a function of

VOUT will cause the switching frequency to increase by the

ratio of VOUT/2.4V for 3.3V and 5V outputs. This is due to

the fact the on time will not increase as VOUT increases

past 2.4V. Therefore, if the nominal switching frequency

is 850kHz, then the switching frequency will increase

to ~1.2MHz for 3.3V, and ~1.7MHz for 5V outputs due

to Frequency = (DC/tON) When the switching frequency

increases to 1.2MHz, then the time period tS is reduced

to ~833 nanoseconds and at 1.7MHz the switching period

reduces to ~588 nanoseconds. When higher duty cycle

conversions like 5V to 3.3V and 12V to 5V need to be

accommodated, then the switching frequency can be

lowered to alleviate the violation of the 400ns minimum

off time. Since the total switching period is tS = tON + tOFF,

tOFF will be below the 400ns minimum off time. A resistor

from the fADJ pin to ground can shunt current away from

the on time generator, thus allowing for a longer on time

and a lower switching frequency. 12V to 5V and 5V to

3.3V derivations are explained in the data sheet to lower

switching frequency and accommodate these step-down

conversions.

Equations for setting frequency for 12V to 5V:

ION = (VIN – 0.7V)/110k; ION = 103μA

frequency = (ION/[2.4V • 10pF]) • DC = 1.79MHz;

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

1MHz frequency or 1μs period is chosen for 12V to 5V.

tON = 0.41 • 1μs ≅ 410ns

tOFF = 1μs – 410ns ≅ 590ns

tON and tOFF are above the minimums with adequate guard

band.

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

ION = (1MHz • 2.4V • 10pF) • (1/0.41) ≅ 58μA. ION current

calculated from 12V input was 103μA, so a resistor from

fADJ to ground = (0.7V/15k) = 46μA. 103μA – 46μA =

57μA, sets the adequate ION current for proper frequency

range for the higher duty cycle conversion of 12V to

5V. Input voltage range is limited to 9V to 16V. Higher

input voltages can be used without the 15k on fADJ. The

inductor ripple current gets too high above 16V, and the

400ns minimum off-time is limited below 9V.

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 = DC • tS, tOFF = off-time of the

switching period; tS = 1/frequency

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

The ~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.

tOFF

PERIOD ts

tON

4600 F21

(DC) DUTY CYCLE =

tON

DC =

tON

FREQ =

tON

VOUT

VIN

Figure 17. LTM4600 Switching Period

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