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MIC3291

1.2MHz PWM White LED Driver with Internal Schottky Diode and Single-Wire Linear Brightness Control

Micrel 

Micrel, Inc.

Shutdown

Whenever a logic level LOW is applied to the DC input

pin for a period greater than or equal to

tSHUTDOWN(1260µs), the MIC3291 will be in power saving

shutdown mode.

Figure 3. Shutdown Timing

Once the device is shutdown, the boost supply is

disabled and the LEDs are turned off. Brightness level

information stored in the MIC3291 prior to shutdown will

be lost.

Programming Pulse Counter Modes

At power up the MIC3291 defaults to count down mode.

In count down mode, the brightness level is decreased

one level by applying two programming pulses, as

shown in Figure 6. The counting mode can be changed

to count up mode by pulling the DC pin LOW for a period

equal to tMODE_UP (100µs to 160µs). The device will

remain in count up mode until its state is changed to

count down mode or by disabling the MIC3291.

MIC3291

Figure 5. Mode Change to Count Down

Programming the Brightness Level

MIC3291 is designed to start driving the output LEDs

(tSTART_UP) in 140µs at the maximum brightness level of

15. After startup, the internal control logic is ready to

decrease the LED brightness upon receiving

programming pulses (negative edges applied to DC pin).

Since MIC3291 starts in count down mode, the

brightness level is decreased one level by applying two

programming pulses, as shown in Figure 6. Each

programming pulse has a high (tPROG_HIGH) and a low

(tPROG_LOW) pulse width that must be between 1µs to

32µs. Note that n+1 number of pulses are needed to

decrease brightness by n level(s) since the first clock

pulse is ignored. Ignoring the first clock pulse is

necessary so mode change (tMODE_UP, tMODE_DOWN) pulses

do not result in adjustments to the brightness level. The

MIC3291 internal circuit can be changed from count

down mode to count up mode and vice versa. The user

may elect to send a mode change as shown in Figure 4

to set the MIC3291 to step up the brightness level with

subsequent programming pulses. For proper operation,

ensure the DC pin has remained HIGH for at least

tDELAY(140µs) before issuing a mode change command.

Figure 4. Mode Change to Count up

To change the state back to count down mode, pull the

DC pin LOW for a period equal to tMODE_DOWN (420µs to

500µs). Now the internal circuitry will remain in count

down mode until changed to count up mode as

described previously.

Figure 6. Brightness Programming Pulses

Multiple brightness levels can be changed together with

multiple programming pulses, as shown in Figure 7.

When issuing multiple brightness level adjustment

commands to the DC pin, ensure both tPROG_LOW and

tPROG_HIGH are within 1µs and 32µs.

September 2010

10

M9999-092810

(408) 944-0800

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