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LT5570

5570f

The high performance RF circuits inside the LT5570 enable

it to handle output ripple as high as 2dB without losing its

power detection accuracy. The ripple can be further reduced

for optimal transient time with an additional RC lowpass

ﬁlter at the output as discussed in the next section.

Output Interface

The output buffer ampliﬁer of the LT5570 is shown in

Figure 9. This push-pull buffer ampliﬁer can source 5mA

current to the load and sink 2.5mA current from the load.

The output impedance is determined primarily by the 100Ω

series resistor connected to the buffer ampliﬁer. This will

prevent any over-stress on the internal devices in case the

output is shorted to ground.

The –3dB bandwidth of the buffer ampliﬁer is about

2.4MHz and the full-scale rise/fall time can be as fast as

175ns. When the output is resistively terminated or open,

the fastest output transient response is achieved when a

large signal is applied to the RF input port. The total rise

time of the LT5570 is about 0.5μs and the total fall time

is 8μs, respectively, for full-scale pulsed RF input power.

The speed of the output transient response is dictated

mainly by the ﬁltering capacitor C1 (at least 22nF) at the

FLTR pin. See the detailed output transient response in

the Typical Performance Characteristics section. When the

RF input has AM content, residual ripple may be present

at the output depending upon the low frequency content

of the modulated RF signal. For example, when 4-carrier

WCDMA is applied at the RF input, ±36mVRMS (about

±1dB) ripple is present at the output. This ripple can be

reduced with a larger ﬁltering capacitor C1 at the expense

of a slower transient response.

APPLICATIONS INFORMATION

Figure 9. Simpliﬁed Circuit Schematic of the Output Interface

100

RSS

OUT

INPUT

VCC

50μA

VOUT

CLOAD

3566 F09

LT5570

Figure 8. Output DC Voltage Variation and

Residual Ripple vs AM Modulation Frequency

AM MODULATION FREQUENCY (MHz)

C1 = 22nF

0.01

5570 F08

0.1

–1

–2

–3

OUTPUT AC RIPPLE

DC VOLTAGE VARIATION

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