LT3477 datasheet(18/26 Pages) AD

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LT3950

Rev. 0

For more information www.analog.com

Inductor Selection

Select an inductor for use with LT3950 that has a satura-

tion current rating greater than 1.7A, additional margin

is recommended. Choose the inductor value based on

desired ripple current given input and output voltage and

switching frequency. Use the equations below to select an

inductor with around 30% peak to peak ripple.

LBOOST >

tSWVIN2

0.3iLEDVLED

1–

VIN

VLED

⎛

⎝⎜

⎞

⎠⎟

LBUCK >

tSWVLED

0.3iLED

1–

VLED

VIN

⎛

⎝⎜

⎞

⎠⎟

Table 5. Inductor Manufacturers

VENDOR

WEBSITE

Sumida

www.sumida.com

Wurth Elektronik

www.we-online.com

Coiltronics

www.cooperet.com

Vishay

www.vishay.com

Coilcraft

www.coilcraft.com

Power PMOS Selection

For the PMOS to be used with PWMTG, select a device

with drain-source voltage rating higher than 65V, addi-

tional margin is recommended. The gate-source voltage

rating should be higher than 10V. The drain current rating

should be sufficient to conduct the full programmed LED

current with some margin. Ensure that the PWMTG drive

voltage of 7.5V will fully enhance the PMOS device. Be

careful when selecting a PMOS to consider the effect that

higher current ratings have on gate charge (Qg). A high Qg

(>20nC) PMOS will slow turn-on and turn-off time, which

can reduce maximum PWM dimming dynamic range.

LED Fault Events

LT3950 provides a variety of fault detection and reporting

functions to aid larger systems in responding to failures.

While LT3950 has many self-protection features, only four

types of faults will be reported to the outside world. Of

these, three (LED Short, Overvoltage and Overcurrent) will

cause the part to stop switching and, if an external PMOS

is used according to instructions in the PWM Dimming

APPLICATIONS INFORMATION

section, disconnect the load from the output capacitor.

The FAULT pin will be asserted. The part will not wait until

the end of the clock cycle to take this action. The LED

Open fault will only result in the FAULT pin being asserted;

switching will continue as normal. A summary table of

each type of fault appears at the end of this section.

Overcurrent fault detection uses the current programming

sense resistor. If the voltage drop across the sense resis-

tor is greater than 700mV (typ.), an overcurrent event

is reported.

While overcurrent senses the actual current into the load,

the LED Short fault senses load voltage. This can allow

the detection of limited failures, such as one or two of the

LEDs in a string becoming shorted. LT3950 senses the

voltage at the FB pin and reports an LED Short fault if that

voltage falls below 300mV (excluding during start-up). It

is important to note that the FB resistor divider must be

in place to use the LED Short fault detection.

To report an LED Open fault, the voltage at the FB pin

must approach 1.2V while the load current falls below

10% of full-scale. It is important to note that the FB resis-

tor divider must be in place to use the LED Open fault

detection.

The Overvoltage fault occurs when the voltage at the FB

pin exceeds 1.3V. Like the LED Open fault, the FB resis-

tor divider must be in place to take advantage of the over

voltage fault protection.

Figure 7.

3950 F07

SOFT START

FB REGULATING

C10

OVERCURRENT

LED SHORT

OVERVOLTAGE

PWM

FAULT

Fault Logic

All four types of faults result in the FAULT pull down turn-

ing on. Since the FAULT pin has an open-drain output,

connecting a resistor from FAULT to INTVCC, VIN, or an

external supply is required for operation. The open-drain

output allows flexibility such as the ability to wire-OR

many parts together to detect faults on an array of devices

with a single digital I/O pin.

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