.

SS

TCAP

SS

C

(nF)

t

(ms)

0 8 x

I

( A)

P

SW

(OCP)

I

5 x I

0.8A

SW

(OCP)

I

3 x I

0.8A

10

TPS65235-1

SLVSDP1 – JANUARY 2017

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Copyright © 2017, Texas Instruments Incorporated

Feature Description (continued)

7.3.3 Boost Converter Current Limit

The boost converter has the cycle-by-cycle peak current limit on the internal Power MOSFET switch to serve as

relationship can be expressed as:

(2)

for the internal Power MOSFET is:

(3)

up to 1 A, especially the VLNB output is high.

7.3.4 Charge Pump

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

charge pump capacitor between VLNB and VCP is about 5.4 V, so the absolute value of the VCP voltage will be

VLNB + 5.4 V.

7.3.5 Slew Rate Control

When LNB output voltage transits from 13.4 V to 18.2 V or 18.2 V to 13.4 V, the cap at pin TCAP controls the

transition time. This transition time makes sure the boost converter output to follow LNB output 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 a 0.8 V above the LNB output.

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

(4)

A 22-nF capacitor generates about 2 ms transition time.

In light load conditions, when LNB output voltage is set from 18.2 V to 13.4 V, the voltage drops very slow, which

causes wrong VOUT_GOOD (Bit 0 at status register 0x02) logic for LNB output voltage detection. TPS65235-1

has integrated a pull down circuit to pull down the output during the transition. This ensures the voltage change

can follow the voltage at TCAP. When the 22-kHz tone signal is superimposing on the LNB output voltage, the

pull down current can also provide square wave instead of a distorted waveforms.

7.3.6 Short Circuit Protection, Hiccup and Overtemperature Protection

The LNB output current limit can be set by an external resistor. When short circuit conditions occur or current

limit is triggered, the output current is clamped at the current limit for 4 ms with LDO on. If the condition retains,

the converter will shut down for 128 ms and then restart. This hiccup behavior prevents IC from being overheat.

The low side MOSFET of the boost converter has a peak current limit threshold which serves as the secondary

protection. If boost converter’s peak current limit is triggered, the peak current will be clamped as high as 3.8 A

starts to drop and output power drops.

Thermal shutdown prevents the chip from operating at exceedingly high temperatures. When the junction

temperature exceeds 160°C, the output shuts down. When the die temperature drops below its lower threshold

typically 140°C, the output is enabled.