LT3932 datasheet(20/28 Pages) AD

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LT8386

Rev. 0

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The internal PWM function of the LT8386 maps the PWM

pin voltages to the PWM dimming ratios as illustrated in

Figure 7. When VPWM is greater than 2V, the PWM dim-

ming ratio is 100% and the PWMTG switch will stay on.

Between 1V and 2V, there are 512 evenly spaced voltage

thresholds corresponding to 512 discrete PWM dimming

ratios from 0% to 100%. The LT8386 provides Preset

PWM dimming ratios that significantly enhance the pre-

cision of the dimming ratio and reduce the noise sensi-

tivity at the PWM pin. Applying 0.3V, 0.5V, and 0.7V at

the PWM pin results in very accurate 1%, 5%, and 10%

PWM dimming ratios, respectively. In between the preset

PWM ranges, the dimming ratio is 0% and the PWMTG

will stay off.

Monitoring LED Current

The ISMON pin provides an amplified and buffered moni-

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

of the internal amplifier is 10, and the speed is fast enough

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

as shown in Figure 8, the ISMON voltage can be filtered

with a resistor-capacitor network to monitor the average

LED current instead.

APPLICATIONS INFORMATION

Figure 7. The Internal PWM Mapping of the LT8386

512 STEPS

VPWM

100%

8386 F07

0.3V

0.5V 0.7V

±50mV ±50mV ±50mV

10%

Figure 8. ISMON Filter Configuration

LT8386

8386 F08

ISMON

ISMON(FILTERED)

CMON

RMON

To avoid excessive start-up times, after the first PWM

pulse, PWMTG will stay on until the SS pin voltage reaches

1.55V or the LED current has reached approximately 10%

of the full-scale current. This expedited start-up applies to

both internal and external PWM operations.

Figure 9. FB Resistor Configuration

LT8386

8386 F09

RFB2

VOUT

RFB1

VOUT(MAX) = 1.2V • 1+

RFB2

RFB1

⎛

⎝

⎜

⎞

⎠

⎟

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

Two resistors should be selected to form a network

between the output voltage and the FB pin as shown in

Figure 9.

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