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LTM4606

4606f

APPLICATIONS INFORMATION

Frequency Adjustment

The LTM4606 is designed to typically operate at 800kHz

across most input conditions. The fSET pin is typically 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 800kHz 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 28V to 5V.

Example for 5V Output

LTM4606 minimum on-time = 100ns;

tON = ((4.8 • 10pf)/IfSET)

LTM4606 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)), where the internal RfSET is 41.2k.

For 28V input operation, IfSET = 227μA. tON = ((4.8 • 10pF)/

IfSET),tON=211ns.Frequency=(VOUT/(VIN•tON))=(5V/(28

• 211ns)) ~ 850kHz. The inductor ripple current begins to

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

across the inductor. The current ripple is ~5A at 20% duty

cycle if the integrated inductor is 1μH. The inductor ripple

current can be lowered at the higher input voltages by add-

ing an external resistor from fSET to ground to increase the

switching frequency. A 4A ripple current is chosen, and the

total peak current is equal to 1/2 of the 4A ripple current

plus the output current. For 5V output, current is limited to

5A, so the total peak current is less than 7A. This is below

the 8A peak speciﬁed value. A 150k resistor is placed from

fSET to ground, and the parallel combination of 150k and

41.2k equates to 32.3k. The IfSET calculation with 32.3k

and 28V input voltage equals 289μA. This equates to a tON

of 166ns. This will increase the switching frequency from

850kHzto~1MHzforthe28Vto5Vconversion.Theminimum

on time is above 100ns at 28V input. Since the switching

frequency is approximately constant over input and output

conditions, then the lower input voltage range is limited to

8V for the 1MHz operation due to the 400ns minimum off

time. Equation: tON = (VOUT/VIN) • (1/Frequency) equates to

a 375ns on time, and a 400ns off time. Figure 18 shows an

operating range of 10V to 28V for 1MHz operation with a

150k resistor to ground, and an 8V to 16V operating range

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

LTM4606 minimum on-time = 100ns;

tON = ((3.3 • 10pF)/IfSET)

LTM4606 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 28V input operation, IfSET =

227μA, tON = ((3.3 • 10pf)/IfSET), tON = 145ns, where the

internal RfSET is 41.2k. Frequency = (VOUT/(VIN • tON)) =

(3.3V/(28 • 145ns)) ~ 810kHz. The minimum on-time and

minimum-off time are within speciﬁcation at 146ns and

1089ns. But the 4.5V minimum input for converting 3.3V

output will not meet the minimum off-time speciﬁcation of

400ns. tON = 905ns, Frequency = 810kHz, tOFF = 329ns.

Solution

Lower the switching frequency at lower input voltages to

allow for higher duty cycles, and meet the 400ns mini-

mum off-time at 4.5V input voltage. The off-time should

be about 500ns with 100ns guard band. The duty cycle

for (3.3V/4.5V) = ~73%. Frequency = (1 – DC)/tOFF or

(1 – 0.73)/500ns = 540kHz. The switching frequency

needs to be lowered to 540kHz at 4.5V input. tON = DC/

frequency, or 1.35μs. The fSET pin voltage compliance

is 1/3 of VIN, and the IfSET current equates to 36μA with

the internal 41.2k. The IfSET current needs to be 24μA for

540kHz operation. A resistor can be placed from VOUT to

fSET to lower the effective IfSET current out of the fSET pin

to 24μA. The fSET pin is 4.5V/3 =1.5V and VOUT = 3.3V,

therefore an 150k resistor will source 12μA into the fSET

node and lower the IfSET current to 24μA. This enables the

540kHz operation and the 4.5V to 28V input operation for

down converting to 3.3V output as shown in Figure 19.

The frequency will scale from 540kHz to 950kHz over this

input range. This provides for an effective output current

of 5A over the input range.

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