LTC6906 Linear Technology, LTC6906 Datasheet - Page 9

• Manufacturer Part Number: LTC6906
• Description: Micropower
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
Achieving the highest accuracy requires controlling po-
tential leakage paths. PC board leakage is aggravated by
both dirt and moisture. Effective cleaning is a good first
step to minimizing leakage, and some PC board manufac-
turers offer high impedance or low leakage processing
options.
Another effective method for controlling leakage is to shunt
the leakage current away from the sensitive node through
a low impedance path. The LTC6906 provides a signal on
the GRD pin for this purpose. Figure 10 shows a PC board
layout that uses the GRD pin and a “guard ring” to absorb
leakage currents. The guard ring surrounds the SET pin
and the end of R
must have no solder mask covering it to be effective. The
GRD pin voltage is held within a few millivolts of the SET
pin voltage, so any leakage path between the SET pin and
the guard ring generates no leakage current.
Figure 10. PC Board Layout with Guard Ring
1
2
3
Figure 9. Supply Current vs Frequency
80
70
60
50
40
30
20
10
0
R
SET
0
OUT
GND
DIV
V
SET
+
MASTER OSCILLATOR FREQUENCY (kHz)
LTC6906
= 2.7V
200
to which it is connected. The guard ring
U
GRD
SET
V
+
400
U
6
5
4
600
6906 F10
÷1
÷3
800
W
÷10
NO LEAKAGE
CURRENT
LEAKAGE
CURRENT
NO SOLDER MASK
OVER THE GUARD RING
GUARD
RING
1000
6906 F09
1200
U
Bypassing the Power Supply
The LTC6906 has on-chip power supply decoupling that
eliminates the need for an external decoupling capacitor in
most cases. Figure 11 shows a simplified equivalent
circuit of the output driver and on-chip decoupling net-
work. When the output driver switches from low to high,
the 800pF capacitor delivers the current needed to charge
the off-chip capacitive load. Within nanoseconds the sys-
tem power supply recharges the 800pF capacitor.
Figure 12 shows a test circuit for evaluating performance
of the LTC6906 with a highly inductive, 330nH power
supply. Figure 13 shows the effectiveness of the
on-chip decoupling network. For C
output waveforms remain well formed.
The extremely low supply current of the LTC6906 allows
operation with substantial resistance in the power supply.
Figure 14 shows a test circuit for evaluating performance
of the LTC6906 with a highly resistive, 100Ω power
supply. Figure 15 shows the effectiveness of the on-chip
decoupling network. For C
waveforms remain well formed. With a 50pF load, a very
small (2.5%) slow tail can be seen on the rising edge. The
output waveform is still well formed even in this case. The
ability of the LTC6906 to operate with a resistive supply
permits supplying power via a CMOS logic gate or
microcontroller pin. Since the LTC6906 has a turn-on time
of less than 200µsec, this technique can be used to enable
the device only when needed and further reduce power
consumption.
C
LOAD
Figure 11. Simplified Equivalent of the Output Driver
and On-Chip Decoupling Circuit
V
+
6
1
2
V
OUT
GND
ESD DIODES
+
300Ω
LTC6906-1
DRIVER
LOAD
f
OUT
= 5pF to 50pF, the output
LOAD
DECOUPLING
NETWORK
20Ω
800pF
= 5pF to 50pF, the
LTC6906
6906 F11
6906fa
9