LT3755 datasheet(18/30 Pages) LINEAR

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LT3922

3922f

For more information www.linear.com/LT3922

applicaTions inForMaTion

However, the LT3922 is capable of PWM dimming even

when an external PWM signal is not available. In this case,

an internal PWM signal with frequency set by a resistor at

the RP pin and duty ratio set by a DC voltage at the PWM

pin will control the PWMTG driver. The RP resistor should

be one of the seven values listed in Table 5. For each of

these values, the PWM frequency is a unique ratio of the

switching frequency.

Table 5. Internal PWM Dimming Frequencies

RATIO

SWITCHING FREQUENCY

2MHz

1MHz

200kHz

28.7k

7.81kHz

3.91kHz

781Hz

47.5k

3.91kHz

1.95kHz

391Hz

76.8k

210

1.95kHz

977Hz

195Hz

118k

211

977Hz

488Hz

97.7Hz

169k

212

488Hz

244Hz

48.8Hz

237k

213

244Hz

122Hz

24.4Hz

332k

214

122Hz

61Hz

12.2Hz

When using the internal PWM signal, set the voltage at

the PWM pin between 1V and 2V. The PWMTG driver will

stay off if PWM is below 1V, and it will stay on if PWM

is above 2V. Between 1V and 2V there are 128 evenly

spaced thresholds corresponding to 128 discrete PWM

duty ratios from 0% to 100%. This range of 1V to 2V has

been chosen so that the PWM voltage may be set using

a potentiometer or a resistor network and the 2V refer-

ence available at the VREF pin. Place a small 1µF ceramic

capacitor near PWM pin to ground.

There are a couple of exceptions to the above rules for

PWM dimming. First, once initiated, the PWM on-time

will last at least four switching cycles regardless of the

signal at the PWM pin and the resistor at the RP pin. This

ensures that the current regulation loop has enough time

to reach equilibrium but still allows for a 5000:1 dimming

ratio when the external PWM frequency is 100Hz and the

switching frequency is 2MHz. Second, to avoid excessive

start-up times, after the first PWM pulse, PWMTG will stay

on until the SS pin voltage reaches 1.7V or the LED current

has reached approximately 10% of the full-scale current.

Figure 7. FB Resistor Configuration

Figure 6. ISMON Filter Configuration

PWM Dimming with Very Long Off Times

ToenhancePWMdimming,theVOUTandVCpinsaredriven

when the PWM pulse (internal or external) is at a logic low

to maintain the charge on the capacitors at those pins.

Consequently, when PWM returns to a logic high state,

the LED current can quickly reach the regulated level even

if PWM was low for a very long time.

Monitoring LED Current

TheISMONpinprovidesanamplifiedandbufferedmonitor

of the voltage between the ISP and ISN pins. The gain of

the internal amplifier is ten, and the speed is fast enough

to track the pulse-width modulated LED current. However,

as shown in Figure 6, the ISMON voltage can be filtered

with a resistor-capacitor network to monitor the average

LED current instead.

The resistor should be at least 10k. The capacitance can

be as large or small as needed without affecting the stabil-

ity of the internal amplifier. For example, when the PWM

frequency is 200Hz, a 10μF capacitor combined with the

10k resistor would limit the ripple on ISMON to 1%.

Selecting the FB Resistors

Tworesistorsshouldbeselectedtoformanetworkbetween

the output voltage and the FB pin as shown in Figure 7.

LT3922

3922 F06

ISMON

ISMON(FILTERED)

RMON

CMON

LT3922

3922 F07

VOUT

RFB2

RFB1

VOUT(MAX) =1.2V • 1+

RFB2

RFB1

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