LT3955 datasheet(13/30 Pages) LINER

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LT3955

3955fb

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applicaTions inFormaTion

When CTRL is higher than 1.2V, the LED current is regu-

lated to:

ILED =

250mV

RLED

The CTRL pin should not be left open (tie to VREF if not

used). The CTRL pin can also be used in conjunction with

a thermistor to provide overtemperature protection for

the LED load, or with a resistor divider to VIN to reduce

output power and switching current when VIN is low.

The presence of a time varying differential voltage signal

(ripple) across ISP and ISN at the switching frequency

is expected. The amplitude of this signal is increased by

high LED load current, low switching frequency and/or a

smaller value output filter capacitor. Some level of ripple

signal is acceptable: the compensation capacitor on the

VC pin filters the signal so the average difference between

ISP and ISN is regulated to the user-programmed value.

Ripple voltage amplitude (peak-to-peak) in excess of

50mV should not cause mis-operation, but may lead to

noticeable offset between the current regulation and the

user-programmed value.

Output Current Capability

An important consideration when using a switch with a

fixed current limit is whether the regulator will be able to

supply the load at the extremes of input and output voltage

range.Severalequationsareprovidedtohelpdeterminethis

capability. Some margin to data sheet limits is included.

For boost converters:

IOUT(MAX) ≤ 2.5A

VIN(MIN)

VOUT(MAX)

For buck mode converters:

IOUT(MAX) ≤ 2.5A

For SEPIC and buck-boost mode converters:

IOUT(MAX) ≤ 2.5A

VIN(MIN)

(VOUT(MAX) + VIN(MIN))

LT3955

VOUT

3955 F02

Figure 2. Feedback Resistor Connection for

Boost or SEPIC LED Driver

These equations assume the inductor value and switch-

ing frequency have been selected so that inductor ripple

current is ~600mA. Ripple current higher than this value

will reduce available output current. Be aware that current

limited operation at high duty cycle can greatly increase

inductor ripple current, so additional margin may be re-

quired at high duty cycle.

If some level of analog dimming is acceptable at minimum

supplylevels,thentheCTRLpincanbeusedwitharesistor

divider to VIN (as shown on page 1) to provide a higher

output current at nominal VIN levels.

Programming Output Voltage (Constant Voltage

Regulation) or Open LED/Overvoltage Threshold

For a boost or SEPIC application, the output voltage can

be set by selecting the values of R3 and R4 (see Figure 2)

according to the following equation:

VOUT = 1.25 •

R3+R4

For a boost type LED driver, set the resistor from the

output to the FB pin such that the expected voltage level

during normal operation will not exceed 1.17V. For an LED

driver of buck mode or a buck-boost mode configuration,

the output voltage is typically level-shifted to a signal with

respect to GND as illustrated in Figure 3. The output can

be expressed as:

VOUT = VBE +1.25 •

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