LTC3548 datasheet(13/30 Pages) LINER

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LT3510

3510fe

maximum duty cycle. The duty cycle is the fraction of time

that the internal switch is on during a clock cycle. Unlike

most ﬁxed frequency regulators, the LT3510 will not switch

off at the end of each clock cycle if there is sufﬁcient volt-

age across the boost capacitor (C3 in Figure 1) to fully

saturate the output switch. Forced switch off for a minimum

time will only occur at the end of a clock cycle when the

boost capacitor needs to be recharged. This operation

has the same effect as lowering the clock frequency for a

ﬁxed off time, resulting in a higher duty cycle and lower

minimum input voltage. The resultant duty cycle depends

on the charging times of the boost capacitor and can be

approximated by the following equation:

DCMAX

where B is 2A divided by the typical boost current from

the Electrical Characteristics.

This leads to a minimum input voltage of:

VIN(MIN)

VOUT + VD

DCMAX

–VD

+ VSW

where VSW is the voltage drop of the internal switch.

Figure 3 shows a typical graph of minimum input voltage

vs load current for the 3.3V and 1.8V application on the

ﬁrst page of this data sheet. The maximum input voltage

is determined by the absolute maximum ratings of the VIN

and BST pins and by the frequency and minimum duty

cycle. The minimum duty cycle is deﬁned as :

DCMIN = tON(MIN) • Frequency

Maximum input voltage as:

VIN(MAX)

VOUT + VD

DCMIN

–VD

+ VSW

Note that the LT3510 will regulate if the input voltage is

taken above the calculated maximum voltage as long as

maximum ratings of the VIN and BST pins are not violated.

However operation in this region of input voltage will exhibit

pulse skipping behavior.

Example:

VOUT = 3.3V, IOUT = 1A, Frequency = 1MHz, Temperature

= 25°C

VSW = 0.1V, B = 40 (from boost characteristics speciﬁca-

tion), VD = 0.4V, tON(MIN) = 200ns

DCMAX

= 98%

VIN(MIN)

3.3

+ 0.4

0.98

– 0.4

+ 0.1= 3.48V

DCMIN

= tMIN(ON) •f = 0.200

VIN(MAX)

3.3

+ 0.4

0.200

– 0.4

+ 0.1= 18.2V

Inductor Selection and Maximum Output Current

A good ﬁrst choice for the inductor value is:

VIN –VOUT

()•VOUT

VIN •f

where f is frequency in MHz and L is in μH.

With this value the maximum load current will be ~2A,

independent of input voltage. The inductor’s RMS current

Figure 3. Minimum Input Voltage vs Load Current

CURRENT (mA)

3.0

3.5

4.0

4.5

5.0

5.5

6.0

100

1000

10000

3510 F03

VOUT = 3.3V

START-UP

RUNNING

APPLICATIONS INFORMATION

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