LT3510EFE(RevC) データシートの表示（PDF） - Linear Technology

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LT3510EFE
(Rev.:RevC)

Monolithic Dual Tracking 2A Step-Down Switching Regulator

Linear Technology 

LT3510

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:

R1=

R2

•

⎛

⎝⎜

VOUT

0.8V

–

1⎞⎠⎟

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) = VOUT + VD • 1

VIN – VSW + VD tON(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.

1600

190

1400

185

FREQUENCY

1200

180

1000

175

800

170

PHASE

600

165

400

160

200

155

100

0

150

20 40 60 80 100 120 140

RESISTANCE (kΩ)

3510 F02

Figure 2. Frequency and Phase vs RT/SYNC Resistance

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 = 3.3 + 0.6 • 1 ~ 750kHz

25 – 0.4 + 0.6 200e-9

RT/SYNC ~ 42k (Figure 2)

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

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

Output

FREQUENCY

1.2MHz

RT/SYNC

20.5k

EFFICIENCY

VVIN1/2 = 12V

79.0%

VIN(MAX)†

16

L*

1.5μH

C* L + C AREA

22μF 63mm2

1.0MHz

26.7k

80.9%

18

2.2μH 47μF 66mm2

750kHz

38.3k

81.2%

22

3.3μH 47μF 66mm2

500kHz

61.9k

82.0%

24

4.7μH 47μF 66mm2

250kHz

133k

83.9%

24

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.

3510fc

11

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