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LT3510

3510fc

APPLICATIONS INFORMATION

Choosing the Output Voltage

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1% resis-

tors according to:

OUT

⎛

⎝⎜

⎞

⎠⎟

•

–

R2 should be 10k or less to avoid bias current errors. Refer-

ence designators refer to the Block Diagram in Figure 1.

Choosing the Switching Frequency

The LT3510 switching frequency is set by resistor R3 in

Figure 1. The RT/SYNC pin is internally regulated at 0.975V.

Setting resistor R3 sets the current in the RT/SYNC pin

which determines the oscillator frequency as illustrated

in Figure 2.

The switching frequency is typically set as high as pos-

sible to reduce overall solution size. The LT3510 employs

techniques to enhance dropout at high frequencies but

efﬁciency and maximum input voltage decrease due to

switching losses and minimum switch on times. The

maximum recommended frequency can be approximated

by the equation:

Frequency Hz

OUT

ON MIN

()

–

•

()

where VD is the forward voltage drop of the catch diode (D1

Figure 2), VSW is the voltage drop of the internal switch,

and tON(MIN) in the minimum on time of the switch, all at

maximum load current.

The following example along with the data in Table 1

illustrates the tradeoffs of switch frequency selection.

Example.

VIN = 25V, VOUT = 3.3V, IOUT = 2.5A,

Temperature = 0°C to 85°C

tON(MIN) = 200ns (85°C from the Typical Performance

Characteristics graph), VD = 0.6V, VSW = 0.4V (85°C)

Max Frequency

–.

•~

33 06

25 04 06

750

200e-9

kkHz

R SYNC

k Figure

T /~

(

)

Input Voltage Range

Once the switching frequency has been determined, the

input voltage range of the regulator can be determined.

The minimum input voltage is determined by either the

LT3510’s minimum operating voltage of ~2.8V, or by its

Figure 2. Frequency and Phase vs RT/SYNC Resistance

Table 1. Efﬁciency and Size Comparisons for Different RRT/SYNC Values. 3.3V

Output

FREQUENCY

RT/SYNC

EFFICIENCY

VVIN1/2 = 12V

VIN(MAX)†

L + C AREA

1.2MHz

20.5k

79.0%

1.5μH

22μF

63mm2

1.0MHz

26.7k

80.9%

2.2μH

47μF

66mm2

750kHz

38.3k

81.2%

3.3μH

47μF

66mm2

500kHz

61.9k

82.0%

4.7μH

47μF

66mm2

250kHz

133k

83.9%

10μH

100μF

172mm2

†VIN(MAX) is deﬁned as the highest input voltage that maintains constant output voltage ripple.

*Inductor and capacitor values chosen for stability and constant ripple current.

RESISTANCE (kΩ)

100

120

140

800

1200

3510 F02

400

100

1600

600

1000

200

1400

170

180

160

150

190

165

175

155

185

FREQUENCY

PHASE

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