LT3477 datasheet(13/26 Pages) AD

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LT3950

Rev. 0

For more information www.analog.com

VOUT = 1+

RFB1

RFB2

⎛

⎝⎜

⎞

⎠⎟

• 1.2V

Figure 2.

LT3950

3950 F02

COUT

VOUT

RFB1

RFB2

Voltage Feedback Network

It is important that the divider be Kelvin connected to the

output capacitor. The resistors RFB1 and RFB2 should be

chosen such that their total value is between 1k and 1M.

For high dynamic range (>10,000:1) PWM dimming, a

total resistance of up to 10M may be appropriate. If the

part is regulating output voltage and the current drops

below 10% of full-scale, an open LED event will be sig-

naled at the FAULT pin.

In some configurations, such as buck mode, where the

load voltage is not directly referred to ground, a level

shifter may be needed to use constant voltage mode

control. An example circuit to accomplish output voltage

control for buck mode and buck-boost mode topologies

is shown below.

VOUT = 2 • VBE + 1.2

RFB1

RFB2

⎛

⎝

⎜

⎞

⎠

⎟

Figure 3.

LT3950

3950 F03

COUT

RFB2

RFB1

10k

LOAD

Voltage Feedback Network with Level Shifter

FB Overvoltage Lockout

As part of the safety feature offered by output voltage

regulation, if the voltage at the FB pin reaches or exceeds

1.3V (typ.), switching stops, PWMTG pulls high to turn

APPLICATIONS INFORMATION

off the disconnect PMOS, and the device reports a fault

at the FAULT pin.

Setting Switching and PWM Dimming Generator

Frequency

To program the switching frequency of LT3950, simply

connect a single resistor to ground from the RT pin. The

table below includes several common RT resistor values

and corresponding switching frequency. It is important to

connect nothing to the RT pin except this resistor.

Table 1. Selected RT Values and Switching Frequency

RT Value (kΩ)

Switching Frequency (MHz)

402

0.3

301

0.4

237

0.5

191

0.6

162

0.7

140

0.8

121

0.9

107

97.6

1.1

80.6

1.2

76.8

1.3

73.2

1.4

66.5

1.5

61.9

1.6

57.6

1.7

52.3

1.8

48.7

1.9

45.3

During start-up, the switching frequency is reduced to

allow sufficient switch OFF time, especially for 2MHz

operation, so that inductor current can ramp below cur-

rent limit when there is minimal voltage across the induc-

tor during the OFF time. This condition is encountered

at start-up or after faults, when the output is still low.

During start-up, or when restarting after faults, switching

frequency will drop to around 40% of its nominal value,

and step up every 16 cycles. For more information about

this, see the section concerning soft-start.

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