RGBHV Driver with EMI Suppression

10

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Vertical and Horizontal SYNC

The MAX9511 has separate, noninverting, vertical and

horizontal sync buffers. Both sync inputs can level-shift

an input as low as 2.3V to a 5V output. Both sync drivers

have hysteresis at their input to prevent “chatter” in their

outputs. The sync output drivers have a 55Ω (typ) out-

used for vertical and horizontal sync in most applica-

tions. Both sync inputs are pulled to DGND through a

47kΩ resistor if the controller’s SYNC source goes high

impedance, or if the inputs are left floating, avoiding

ambiguous output conditions.

EMI can result from rapid transitions of the sync or the

video signals. To reduce the rise and fall times of the

sync signal, additional capacitance may be added to

the sync outputs. Adding additional capacitance may

require “recentering” the display.

Display Data Channel (DDC)

The MAX9511 bidirectional display data channel (DDC)

level translator allows for a lower voltage video controller

logic to operate with a higher voltage external monitor

the DDC output thresholds. Two Schottky-clamped npn

transistors shift the lower level DDC inputs to higher

logic-level outputs.

DDC_CLK_OUT and DDC_DATA_OUT are pulled to

conditions when left floating. At shutdown, DDC inputs

can still respond to external commands.

Slew-Rate Limiting

The MAX9511 outputs are slew-rate limited to reduce

EMI. Slew-rate limiting affects the large-signal bandwidth

(LSBW) more than the small-signal bandwidth (SSBW),

and can be scaled according to the following formula:

and LSBW(-3dB) is the -3dB bandwidth.

The slew rate of the MAX9511 is controlled by a resistor

ied to optimize the EMI suppression to the display reso-

range is approximately 7kΩ for maximum slew rate and

50kΩ for minimum slew rate (see Figure 3). Slew-rate

limiting can be approximated by:

Shutdown

The MAX9511 features a low-power shutdown mode for

battery-powered/portable applications. Shutdown

reduces the quiescent current of the video and sync dri-

vers. Connecting SHDN to ground (DGND) disables the

outputs and places the MAX9511 into a low-power shut-

down mode. SHDN has a 330kΩ (typ) internal pulldown

operation.

Applications Information

Customizing Slew Rates for

Different Resolutions

When the MAX9511 connects to devices of different

resolutions, different slew rates should be used. The

for a resolution, the MAX9511 minimizes the EMI and

optimizes the video output quality. Shown are two con-

ues for different video resolutions.

Figure 4 shows how to customize slew rates for three res-

olutions. This circuit provides three predetermined slew

The combination is controlled by a digital command from

the video controller through a switch. This requires that

the sample clock rates used by different resolutions are

are set for the highest resolution.

(/

)

SR

R

Vs

=

⎛

⎝

⎜

⎞

⎠

⎟

1030

7000

RX

µ

()

LSBW

dB

R

V

−

=

××

3

2

S

OUT

π

40

30

10

20

200

400

600

800

1000

1200

1400

1600

0

050

Figure 3. Slew Rate vs. RRX