LT5570
14
5570f
Figure 10. Residual ripple, Output Transient
Times with Output Low-pass Filter
Figure 11. Enable Pin Simplifi ed Circuit
Since the output amplifi er of the LT5570 is capable of driv-
ing an arbitrary capacitive load, the residual ripple can be
fi ltered at the output with a series resistor R
SS
and a large
shunt capacitor C
LOAD
. See Figure 9. This lowpass fi lter
also reduces the output noise by limiting the output noise
bandwidth. When this RC network is designed properly,
a fast output transient response can be maintained with
reduced residual ripple. We can estimate C
LOAD
with an
output voltage swing of 1.8V at 2140MHz. In order that
the maximum 2.5mA sinking current not limit the fall time
(about 8μS), C
LOAD
can be chosen as follows.
C
LOAD
= 2.5mA • approximate additional time/1.8V
= 2.5mA • 0.25μs/1.8V = 347pF
Once C
LOAD
is determined, R
SS
can be chosen properly
to form a RC lowpass fi lter with a corner frequency of
2π/(R
SS
• C
LOAD
). Using 4-carrier W-CDMA as an example,
Figure 10 shows the residual ripple is reduced to half from
36mV
RMS
with R
SS
= 4.7k and C
LOAD
= 330pF, while the
fall time is slightly increased to 8.8μS.
EN
V
CC
5570 F11
100k 100k
APPLICATIONS INFORMATION
In general, the rise time of the LT5570 is much shorter
than the fall time. However, when the output RC fi lter is
used, the rise time is dominated by the time constant of
this fi lter. Accordingly, the rise time becomes very similar
to the fall time.
Enable Interface
A simplifi ed schematic of the EN Pin interface is shown
in Figure 11. The enable voltage necessary to turn on the
LT5570 is 2V. To disable or turn off the chip, this voltage
should be below 1V. It is important that the voltage applied
to the EN pin should never exceed V
CC
by more than 0.3V.
Otherwise, the supply current may be sourced through
the upper ESD protection diode connected at the EN pin.
Under no circumstances should voltage be applied to the
EN Pin before the supply voltage is applied to the V
CC
pin.
If this occurs, damage to the IC may result.
TIMES (μs)
0
V
OUT
(V)
RESIDUAL RIPPLE (mV)
1.2
1.6
2.0
80
5570 F10
0.8
0.4
1.0
1.4
1.8
0.6
0.2
0
40
120
200
–40
–120
0
80
160
–80
–160
–200
2010
4030
60 70 90
50
100
AT 2140MHZ, P
IN
= 10dBm
RF
PULSE
OFF
RF
PULSE
OFF
RF PULSE ON
WITH FILTERING
WITHOUT
FILTERING
