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LT3954IUHE

40VIN LED Converter with Internal PWM Generator

Linear Technology 

LT3954

PIN FUNCTIONS

SYNC (Pin 1): Frequency Synchronization Pin. Used to

synchronize the internal oscillator to an outside clock. If

this feature is used, an RT resistor should be chosen to

program a switching frequency 20% slower than SYNC

pulse frequency. Tie the SYNC pin to PWMOUT if this

feature is not used.

EN/UVLO (Pin 2): Enable and Undervoltage Detect Pin. An

accurate 1.22V falling threshold with externally program-

mable hysteresis causes the switching regulator to shut

down when power is insufficient to maintain output regu-

lation. Above the 1.24V (typical) rising enable threshold

(but below 2.5V), EN/UVLO input bias current is sub-μA.

Below the 1.22V (typical) falling threshold, an accurate

2.2μA (typical) pull-down current is enabled so the user

can define the rising hysteresis with the external resistor

selection. An undervoltage condition causes the switch to

turn off and the PWMOUT pin to transition low and resets

soft-start. Tie to 0.4V, or less, to disable the device and

reduce VIN quiescent current below 1μA. Can be tied to

VIN through a 100k resistor.

INTVCC (Pin 3): Current limited, low dropout linear regula-

tor regulates to 7.85V (typical) from VIN. Supplies internal

loads, SW and PWMOUT drivers. Must be bypassed with

a 1µF ceramic capacitor placed close to the pin and to the

exposed pad GND of the IC.

VIN (Pin 6): Power Supply for Internal Loads and INTVCC

Regulator. Must be locally bypassed with a 0.22µF (or

larger) low ESR capacitor placed close to the pin.

GNDK (Pin 12): Kelvin Connection Pin between PGND

and GND. Kelvin connect this pin to the GND plane close

to the IC. See the Board Layout section.

PGND (Pins 13 to 17): Source Terminal Switch and the

GND Input to the Switch Current Comparator.

PWMOUT (Pin 23): Buffered Version of PWM Signal for

Driving LED Load Disconnect NMOS or Level Shift. This

pin also serves in a protection function for the FB over-

voltage condition—will toggle if the FB input is greater

than the FB regulation voltage (VFB) plus 60mV (typical).

The PWMOUT pin is driven from INTVCC. Use of a FET

with gate cut-off voltage higher than 1V is recommended.

FB (Pin 25): Voltage Loop Feedback Pin. FB is intended for

constant-voltage regulation or for LED protection and open

LED detection. The internal transconductance amplifier with

output VC will regulate FB to 1.25V (nominal) through the

DC/DC converter. If the FB input exceeds the regulation

voltage, VFB, minus 50mV and the voltage between ISP

and ISN has dropped below the C/10 threshold of 25mV

(typical), the VMODE pull-down is asserted. This action

may signal an open LED fault. If FB is driven above the

FB overvoltage threshold, the PWMOUT pin will be driven

low and the internal power switch is turned off, to protect

the LEDs from an overcurrent event. Do not leave the FB

pin open. If not used, connect to GND.

ISN (Pin 27): Connection Point for the Negative Terminal

of the Current Feedback Resistor. The constant output

current regulation can be programmed by ILED = 250mV/

RLED when CTRL > 1.2V or ILED = (CTRL – 100mV)/(4 •

RLED). If ISN is greater than INTVCC, input bias current

is typically 20μA flowing into the pin. Below INTVCC, ISN

bias current decreases until it flows out of the pin.

ISP (Pin 28): Connection Point for the Positive Terminal of

the Current Feedback Resistor. Input bias current depends

upon CTRL pin voltage. When it is greater than INTVCC it

flows into the pin. Below INTVCC, ISP bias current decreases

until it flows out of the pin. If the difference between ISP

and ISN exceeds 600mV (typical), then an overcurrent

event is detected. In response to this event, the switch is

turned off and the PWMOUT pin is driven low to protect

the switching regulator, a 1.5mA pulldown on PWM and

a 9mA pulldown on the DIM/SS pin are activated for 4µs.

VC (Pin 30): Transconductance Error Amplifier Output

Pin Used to Stabilize the Switching Regulator Control

Loop with an RC Network. The VC pin is high impedance

when PWM is low. This feature allows the VC pin to store

the demand current state variable for the next PWM high

transition. Connect a capacitor between this pin and GND;

a resistor in series with the capacitor is recommended for

fast transient response.

3954fa

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For more information www.linear.com/3954

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