LTC6992-3 datasheet(23/34 Pages) AD

23 / 34 page

LTC6992-1/LTC6992-2/

LTC6992-3/LTC6992-4

Rev. D

For more information www.analog.com

Power Supply Current

The power supply current varies with frequency, supply

voltage and output loading. It can be estimated under any

condition using the following equation:



If NDIV = 1 (DIVCODE = 0 or 15):

IS(TYP) ≈ V+ • fOUT • 39pF +CLOAD

(

)

+

V+

320kΩ

V+ •Duty Cycle

RLOAD

+ 2.2 •ISET + 85µA

If NDIV > 1 (DIVCODE = 1 or 14):

IS(TYP) ≈ V+ • NDIV • fOUT •27pF

+ V+ • fOUT • 28pF +CLOAD

(

)

+

V+

320kΩ

V+ •Duty Cycle

RLOAD

+ 2.6 •ISET + 90µA

SUPPLY BYPASSING AND PCB LAYOUT GUIDELINES

The LTC6992 is a 2.4% accurate silicon oscillator when

used in the appropriate manner. The part is simple to use

and by following a few rules, the expected performance

is easily achieved. Adequate supply bypassing and proper

PCB layout are important to ensure this.

Figure 12 shows example PCB layouts for both the TSOT-

23 and DFN packages using 0603 sized passive compo-

nents. The layouts assume a two layer board with a ground

plane layer beneath and around the LTC6992. These lay-

outs are a guide and need not be followed exactly.

APPLICATIONS INFORMATION

1. Connect the bypass capacitor, C1, directly to the V+ and

GND pins using a low inductance path. The connection

from C1 to the V+ pin is easily done directly on the top

layer. For the DFN package, C1’s connection to GND

is also simply done on the top layer. For the TSOT-23,

OUT can be routed through the C1 pads to allow a good

C1 GND connection. If the PCB design rules do not

allow that, C1’s GND connection can be accomplished

through multiple vias to the ground plane. Multiple

vias for both the GND pin connection to the ground

plane and the C1 connection to the ground plane are

recommended to minimize the inductance. Capacitor

C1 should be a 0.1μF ceramic capacitor.

2. Place all passive components on the top side of the

board. This minimizes trace inductance.

3. Place RSET as close as possible to the SET pin and

make a direct, short connection. The SET pin is a cur-

rent summing node and currents injected into this pin

directly modulate the operating frequency. Having a

short connection minimizes the exposure to signal

pickup.

4. Connect RSET directly to the GND pin. Using a long path

or vias to the ground plane will not have a significant

affect on accuracy, but a direct, short connection is

recommended and easy to apply.

5. Use a ground trace to shield the SET pin. This provides

another layer of protection from radiated signals.

6. Place R1 and R2 close to the DIV pin. A direct, short

connection to the DIV pin minimizes the external

signal coupling.

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