LT3956 데이터 시트보기 (PDF) - Linear Technology

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LT3956

80VIN, 80V OUT Constant-Current, Constant-Voltage Converter

Linear Technology 

LT3956

Pin Functions

NC: No Internal Connection. These pins may be left floating

or connected to an adjacent pin.

EN/UVLO: Shutdown and Undervoltage Detect Pin. An

accurate 1.22V falling threshold with externally program-

mable hysteresis detects when power is OK to enable

switching. Rising hysteresis is generated by the external

resistor divider and an accurate internal 2.1µA pull-down

current. Above the 1.24V (nominal) threshold (but below

6V), EN/UVLO input bias current is sub-µA. Below the

falling threshold, a 2.1µA pull-down current is enabled so

the user can define the hysteresis with the external resis-

tor selection. An undervoltage condition resets soft-start.

Tie to 0.4V, or less, to disable the device and reduce VIN

quiescent current below 1µA.

INTVCC: Regulated supply for internal loads, GATE driver

and PWMOUT driver. Supplied from VIN and regulates to

7.15V (typical). INTVCC must be bypassed with a 4.7µF

capacitor placed close to the pin. Connect INTVCC directly

to VIN if VIN is always less than or equal to 7V.

GND: Ground. The exposed pad, Pin 37, is ground and

must be soldered directly to the ground plane.

VIN: Input Supply Pin. Must be locally bypassed with

a 0.22µF (or larger) capacitor to PGND placed close to

the IC.

SW: The exposed pad, Pin 38, is the drain of the switch-

ing N-channel MOSFET and must be connected to the

external inductor.

PGND: Source terminal of switch and the GND input to

the switch current comparator. Kelvin connect to the GND

plane close to the IC using Pin 12. Pins 13 to 17 should be

connected externally to the PGND terminals of components

in the switching path. See the Board Layout section.

PWMOUT: Buffered Version of the PWM Signal. This pin is

used to drive the LED load disconnect N-channel MOSFET

or level shift. This pin also serves in a protection function

for the FB overvoltage 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 MOSFET with gate cut-off voltage higher than

1V is recommended.

FB: Voltage Loop Feedback Pin. FB is intended for con-

stant-voltage regulation or for LED protection/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 is regulating the loop, the

VMODE pull-down is asserted. This action may signal an

open LED fault. If FB is driven above the FB threshold (by

an external power supply spike, for example), the VMODE

pull-down will be de-asserted and the PWMOUT pin will

be driven low to protect the LEDs from an overcurrent

event. Do not leave the FB pin open. If not used, connect

to GND.

ISN: Connection point for the negative terminal of the

current feedback resistor. If ISN is greater than 2.9V, the

LED current can be programmed by ILED = 250mV/RLED

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

if VCTRL < 1V. Input bias current is typically 20µA. Below

3V, ISN is an input to the short-circuit protection feature

that forces GATE to 0V if ISP exceeds ISN by more than

350mV (typ).

ISP: Connection point for the positive terminal of the current

feedback resistor. Input bias current for this pin depends

on CTRL pin voltage, as shown in the Typical Performance

Characteristics. ISP is an input to the short-circuit protec-

tion feature when ISN is less than 3V.

VC: Transconductance Error Amplifier Output Pin. This pin

is used to stabilize the voltage loop with an RC network.

This pin is high impedance when PWM is low, a feature

that stores 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.

CTRL: Current Sense Threshold Adjustment Pin. Regula-

ting threshold V(ISP – ISN) is 0.25 • VCTRL plus an offset

for 0V < VCTRL < 1V. For VCTRL > 1.2V the current sense

threshold is constant at the full-scale value of 250mV. For

1V < VCTRL < 1.2V, the dependence of the current sense

threshold upon VCTRL transitions from a linear function

to a constant value, reaching 98% of full-scale value by

VCTRL = 1.1V. Connect CTRL to VREF for the 250mV default

threshold. Do not leave this pin open.

3956f



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