LTM4601A datasheet(20/28 Pages) LINER

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LTM4601A/LTM4601A-1

4601afb

Frequency Adjustment

The LTM4601A is designed to typically operate at 850kHz

across most input conditions. The fSET pin is normally left

open or decoupled with an optional 1000pF capacitor. The

switching frequency has been optimized for maintaining

constant output ripple noise over most operating ranges.

The 850kHz switching frequency and the 400ns minimum

off time can limit operation at higher duty cycles like 5V to

3.3V, and produce excessive inductor ripple currents for

lower duty cycle applications like 20V to 5V. The 5V and

3.3V drop out curves are modiﬁed by adding an external

resistor on the fSET pin to allow for lower input voltage

operation, or higher input voltage operation.

Example for 5V Output

LTM4601A minimum on time = 100ns;

tON = ((4.8 • 10pf)/IfSET)

LTM4601A minimum off time = 400ns; tOFF = t – tON,

where t = 1/Frequency

Duty Cycle = tON/t or VOUT/VIN

Equations for setting frequency:

IfSET = (VIN/(3 • RfSET)), for 20V operation, IfSET = 170μA,

tON = ((4.8 • 10pF)/IfSET), tON = 282ns, where the internal

RfSET is 39.2k. Frequency = (VOUT/(VIN • tON)) = (5V/(20

• 282ns)) ≈ 886kHz. The inductor ripple current begins

to get high at the higher input voltages due to a larger

voltage across the inductor. This is noted in the “Induc-

tor Ripple Current vs Duty Cycle” graph (Figure 3) where

IL ≈ 10A at 25% duty cycle. The inductor ripple current

can be lowered at the higher input voltages by adding an

external resistor from fSETtogroundtoincreasetheswitch-

ing frequency. An 8A ripple current is chosen, and the total

peak current is equal to 1/2 of the 8A ripple current plus

the output current. The 5V output current is limited to 8A,

so the total peak current is less than 12A. This is below the

14A peak speciﬁed value. A 100k resistor is placed from

fSET to ground, and the parallel combination of 100k and

39.2k equates to 28k. The IfSET calculation with 28k and

20V input voltage equals 238μA. This equates to a tON of

200ns. This will increase the switching frequency from

~886kHz to ~1.25MHz for the 20V to 5V conversion. The

minimum on time is above 100ns at 20V input. Since

the switching frequency is approximately constant over

input and output conditions, then the lower input voltage

range is limited to 10V for the 1.25MHz operation due to

the 400ns minimum off time. Equation: tON = (VOUT/VIN)

• (1/Frequency) equates to a 400ns on time, and a 400ns

off time. The “VIN to VOUT Step-Down Ratio” curve reﬂects

an operating range of 10V to 20V for 1.25MHz operation

with a 100k resistor to ground, and an 8V to 16V operation

for fSET ﬂoating. These modiﬁcations are made to provide

wider input voltage ranges for the 5V output designs while

limiting the inductor ripple current, and maintaining the

400ns minimum off time.

Example for 3.3V Output

LTM4601A minimum on time = 100ns;

tON = ((3.3 • 10pF)/IfSET)

LTM4601A minimum off time = 400ns;

tOFF = t – tON, where t = 1/Frequency

Duty Cycle (DC) = tON/t or VOUT/VIN

Equations for setting frequency:

IfSET = (VIN/(3 • RfSET)), for 20V operation, IfSET = 170μA,

tON = ((3.3 • 10pf)/IfSET), tON = 195ns, where the internal

RfSET is 39.2k. Frequency = (VOUT/(VIN • tON)) = (3.3V/(20

• 195ns)) ≈ 846kHz. The minimum on time and minimum

off time are within speciﬁcation at 195ns and 980ns. The

4.5V minimum input for converting 3.3V output will not

meet the minimum off-time speciﬁcation of 400ns. tON =

868ns, Frequency = 850kHz, tOFF = 315ns.

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