ss

cad

ss

C (nF)

T

(ms)

0.5

I ( A)

=

´

m

1.178

SEL

SEL

R

(k )

124.11 I

(A)

-

W =

´

500

450

400

350

300

250

200

150

100

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

y = 124.11x

-1.178

ISEL (A)

TPS65233-1

SLVSD66 – SEPTEMBER 2015

www.ti.com

Feature Description (continued)

If two or more set top box LNB outputs are connected together, one output voltage could be set higher than

others. The output with lower set voltage would be effectively turned off. Once the voltage drops to the set level,

the LNB output with lower set output voltage will return to normal conditions.

7.3.2 Linear Regulator and Current Limit

The linear regulator is used to generate the 22-kHz tone signal by changing the reference voltage. The linear

regulator features low drop out voltage to minimize power loss while keeping enough head room for the 0.68-V,

22-kHz tone. It also implements a tight current limit for over current protection. The current limit is set by an

external resistor connected to the ISEL pin. The curve below shows the relationship between the current limit

threshold and the resistor value.

Figure 8. Linear Regulator Current Limit vs Resistor

(1)

A 280-k

7.3.3 Charge Pump

The charge pump circuitry generates a voltage to drive the NMOS of the linear regulator. One end the charge

pump capacitor is connected to the output of the boost converter. The voltage on the charge pump capacitor is

about 6.25 V.

7.3.4 Slew Rate Control

When LNB output voltage transits from 13 V to 18 V or vice versa, the capacitor at pin TCAP controls the

transition time. This transition is to make sure the boost converter can follow the voltage change. Usually boost

converter has low bandwidth and can’t response fast. The voltage at TCAP acts as the reference voltage of the

linear regulator. The boost converter’s reference is also based on TCAP with additional fixed voltage to generate

0.8 V above the output.

The charging and discharging current is 10 µA, thus the transition time can be calculated as:

(2)

A 22-nF capacitor generates a 1.1-ms transition time.

In light load conditions, when LNB output voltage is set from 18 to 13 V, the voltage might drops very slow, which

might cause wrong logic detection at LNB side. The TPS65233-1 has an integrated pull down circuit to pull down

the output during the transition. This ensures the voltage change can follow the voltage at TCAP. Meanwhile,

when the 22-kHz tone signal is superimposing on the LNB output voltage, the pull down current can also provide

a square wave instead of distorted waveforms, which could cause another detection problem.

10

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