Application Notes

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Configuring the PTB4850x & PTB4851x DC/DC

Converters for DSL Applications

When operated as a pair, the PTB4850x and PTB4851x

converters are specifically designed to provide all the

required supply voltages for powering xDSL chipsets.

The PTB4850x produces two logic voltages. They include

a 3.3-V source for logic and I/O, and a low-voltage for

powering a digital signal processor core. The PTB4851x

produces a balanced pair of complementary supply voltages

that is required for the xDSL transceiver ICs. When used

together in these types of applications, the PTB4850x and

PTB4851x may be configured for power-up sequencing,

and also synchronized to a common switch conversion

frequency. Figure 2-1 shows the required cross-connects

between the two converters to enable these two features.

Switching Frequency Synchronization

Unsynchronized, the difference in switch frequency

introduces a beat frequency into the input and output

AC ripple components from the converters. The beat

frequency can vary considerably with any slight variation

in either converter’s switch frequency. This results in a

variable and undefined frequency spectrum for the ripple

waveforms, which would normally require separate filters

at the input of each converter. When the switch frequency

of the converters are synchronized, the ripple components

are constrained to the fundamental and higher. This

simplifies the design of the output filters, and allows a

common filter to be specified for the treatment of input

ripple.

Figure 2-1; Example of PTB4850x & PTB4851x Modules Configured for DSL Applications

–V

TCVR

PTB48500A

+Vin

Enable

–Vin

Vo

1

Vo

2

COM

Sync Out

EN Out

Vo

POR

10

1

3

5

42

7

8

6

9

PTB48510A

+Vin

Enable

–Vin

+V

o

–V

o

COM

Sync In

±V

5

1

3

4

2

6

8

7

V

CCIO

V

CORE

+V

TCVR

–48 V RTN

–48 V

Input

Filter

+

–

POR

Power-Up Sequencing

The desired power-up sequence for the AC7 supply volt-

ages requires that the two logic-level voltages from the

PTB4850x converter rise to regulation prior to the two

complementary voltages that power the transceiver ICs.

This sequence cannot be guaranteed if the PTB4850x

and PTB4851x are allowed to power up independently,

especially if the 48-V input voltage rises relatively slowly.

To ensure the desired power-up sequence, the “EN Out”

pin of the PTB4850x is directly connected to the active-

low “Enable” input of the PTB4851x (see Figure 2-1).

This allows the PTB4850x to momentarily hold off the

outputs from the PTB4851x until the logic-level voltages

have risen first. Figure 2-2 shows the power-up wave-

forms of all four supply voltages from the schematic of

Figure 2-1.

Figure 2-2; Power-Up Sequencing Waveforms

PTB4850x & PTB4851x

V

(1 V/Div)

V

(1 V/Div)

CORE

CCIO

+V

(5 V/Div)

–V

(5 V/Div)

TCVR

TCVR

HORIZ SCALE: 10 ms/Div