PTB48520W Datasheet, PDF (10/18 Pages) Texas Instruments – 25-A, 48-V INPUT ISOLATED DC/DC CONVERTER WITH AUTO-TRACK™ SEQUENCING

PTB48520W
SLTS233A – NOVEMBER 2004 – REVISED APRIL 2005
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and enables the output of the converter. The converter produces a regulated output voltage within 50 ms.
Figure 7 shows the output response of the converter after Q1 is turned on. The turnon of Q1 corresponds to the
drop in the Q1 Vds waveform. Although the output voltage rise time is short (<10 ms), the indicated delay time
(t_d) varies depending on the input voltage and the module’s internal timing. The output voltage of the module was
set to 3.3 V. The waveforms were measured with 48-Vdc input voltage and a 10-A resistive load.
V_OBus (1 V/div)
I_I (0.5 A/div)
t_d
Q1 V_ds (10 V/div)
t − 5 ms/div
Figure 7. Output Enable Power-Up Characteristic
Sequenced Power Up with POL Modules
Overview
The dc/dc converter has two outputs, V_O Seq and V_O Bus. V_O Bus is the main output from the converter. V_O Seq
is an output that is derived from V_O Bus and can be sequenced with other supply voltages during power up. Both
outputs are regulated to the same set-point voltage, except that the rise in the V_O Seq output is controlled by the
pin called Track, and delayed during power-up events. The delay allows the converter to both power and
sequence with one or more non-isolated, 3.3-V input, Auto-Track compatible modules¹. The rise of all output
voltages is coordinated using a control signal common to the Track inputs of all Auto-Track compliant modules.
The hold-off delay built into the PTB48520W holds the track control signal low to comply with the power-up
requirements of the downstream non-isolated modules.
Auto-Track Features
Figure 8 shows a block diagram of the converter Auto-Track features. During power up, V_O Bus (pin 9) rises
promptly, whenever the converter is connected to a valid input source and its output is enabled. V_O Seq (pin 10)
is the Auto-Track compatible output that is derived from V_O Bus but controlled by the voltage presented at the
Track input (pin 4). The control relationship is on a volt-for-volt basis, and is active from 0 V up to a voltage just
below the V_O Bus output. Between these two limits, the voltage at V_O Seq follows that at the Track input. Once
the Track input is at the V_O Bus voltage, raising it higher has no further effect. The voltage at V_O Seq cannot go
higher than V_O Bus, and if connected to +Sense (pin 8), then regulates at the set-point voltage.²
The control relationship between V_O Seq and the Track input is the same as other Auto-Track compatible
outputs, across all module types. By connecting the Track input of the converter to the Track inputs of other
Auto-Track modules, all the associated output voltages can be made to follow the same rising control voltage
during power-up transitions.³ A suitable rising voltage is produced by the Track input internal R-C time constant.
An external ramp waveform^{4, 5} may also be used.
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