Datasheet

LTC6990
20
6990fc
For more information www.linear.com/LTC6990
APPLICATIONS INFORMATION
Modulation Bandwidth and Settling Time
The LTC6990 will respond to changes in I
SET
up to a –3dB
bandwidth of 0.4 • f
OUT
(see Figure 15). This makes it easy
to stabilize a feedback loop around the LTC6990, since it
does not introduce a low-frequency pole.
Settling time depends on the master oscillator frequency.
Following a 2x or 0.5x step change in I
SET
, the output
frequency takes approximately six master clock cycles
(6 • t
MASTER
) to settle to within 1% of the final value. An
example is shown in Figure 16.
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:
I
S(TYP)
≈ V
+
• f
MASTER
• 7pF + V
+
• f
OUT
•(13pF +C
LOAD
)
+
V
+
480kΩ
+
V
+
2 •R
LOAD
+ 1.75 •I
SET
+ 50µA
The equation is also valid for OE = 0 (output disabled),
with f
OUT
= 0Hz.
Figure 15. Modulation Frequency Response
Figure 16. Settling Time
f
MOD
/f
OUT
(Hz/Hz)
0.1
∆f
OUT
(f
MOD
)/∆f
OUT
(DC) (dB)
0
–20
–30
–10
–40
1
6990 F15
10
V
CTRL
= 0.536V + 0.278V
• SIN(2π•f
MOD
•t)
f
OUT
=18.75kHz ±10%
–3dB AT 0.4•f
OUT
R
SET
= 200k
R
VCO
= 464k
DIVCODE = 4(÷16)
10µs/DIV
6990 F16
V
CTRL
2V/DIV
f
OUT
50kHz/DIV
OUT
2V/DIV
V
+
= 3.3V, DIVCODE = 0
R
SET
= 200k, R
VCO
= 464k
f
OUT
= 175kHz AND 350kHz