TDF8553J datasheet(13/47 Pages) NXP

13 / 47 page

TDF8553J_1

Objective data sheet

Rev. 01 — 3 December 2008

13 of 47

NXP Semiconductors

TDF8553J

× 50 Watt power ampliﬁer and multiple voltage regulator

8.6.3 Backup function

The backup function is implemented by a switch function, which behaves like an ideal

diode between pins VP and BUCAP; the forward voltage of this ideal diode depends on

the current ﬂowing through it. The backup function supplies regulator 2 during brief

periods when no supply voltage is present on pin VP. It requires an external capacitor to

be connected to pin BUCAP and ground. When the supply voltage is present on pin VP

this capacitor will be charged to a level of VP − 0.3 V. When the supply voltage is absent

from pin VP, this charge can then be used to supply regulator 2 for a brief period (tbackup)

calculated using the formula:

Example: VP = 14.4 V, VREG2 (voltage on pin REG2) = 3.3 V, RL =1kΩ and

Cbackup = 100 µF provides a tbackup of 321 ms.

When an overvoltage condition occurs, the voltage on pin BUCAP is limited to

approximately 24 V; see Figure 5.

8.6.4 Power switches

There are two power switches that provide an unregulated DC voltage output for

ampliﬁers and an aerial motor respectively. The switches have internal protection for

over-temperature conditions and are activated by setting bits D2 and D3 of instruction

byte IB1 to logic 1.

In the ON state, the switches have a low impedance to the battery voltage. When the

battery voltage is higher than 22 V, the switches are switched off. When the battery

voltage is below 22 V the switches are set to their original condition.

The power switches have built-in surge protection to be able to absorb energy from

switching inductive or capacitive external loads. This surge protection is implemented in

such a way that in case no supply (VP) is present the supply line will not be charged from

a possible external source connected to a power switch output.

Fig 5.

Backup capacitor function

backup

REG2

0.5

–

()

–

REG2

-----------------------------------------------





(V)

t (s)

tbackup

out of regulation

VREG2

VBUCAP

001aai693

backup

REG2

0.5

–

()

–

load

------------------------------------------------





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