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PTB78520WAS Datasheet(PDF) 9 Page - Texas Instruments |
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PTB78520WAS Datasheet(HTML) 9 Page - Texas Instruments |
9 / 15 page Application Notes For technical support and further information visit http://power.ti.com PTB78520W Configuring the PTB78520 DC/DC Converter to Power-Up Sequence with POL Modules Overview The PTB78520 DC/DC converter has two outputs, ‘VO Seq’ and ‘VO Bus’. ‘VO Bus’ is the main output from the converter. ‘VO Seq’ is an output that is derived from ‘VO 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 ‘VO Seq’ output is controlled by a pin called ‘Track’, and delayed during power-up events. This delay allows the PTB78520W to both power and sequence with one or more non-isolated, 3.3-V input, Auto-Track compatible modules 1. In these applications, the PTB78520W incor- porates the necessary timing to coordinate the rise of all sequenced outputs using a common track control signal. The hold-off delay time also complies with the power-up requirements of the downstream non-isolated modules, without the need for additional components. PTB78520W Auto-Track Features Figure 2-1 shows a block diagram of the PTB78520W Auto-Track features. During power up, ‘VO Bus’ (pin 9) rises promptly, whenever the converter is connected to a valid input source and its output is enabled. ‘VO Seq’ (pin 10) is the Auto-Track compatible output that is derived from ‘VO Bus’ but directly controlled by the voltage pre- sented 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 ‘VO Bus’ output. Between these two limits, the voltage at ‘VO Seq’ will follow that at the ‘Track’ input. However, once the Track input is at the ‘VO Bus’ voltage, raising it higher has no further effect. The volt- age at ‘VO Seq’ cannot go higher than ‘VO Bus’, and if it is connected to ‘+Sense’ (pin 8), it will then regulate at the set-point voltage. 2 The control relationship between ‘VO 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 PTB78520W to the Track input of other Auto-Track compatible modules, the output voltages can be made to follow a common signal during power-up transitions. 3 Each Track input produces a suitable track control signal from an internal R-C time constant. An input signal can also be provided from an externally generated ramp wave- form. 4, 5 The Track input of the PTB78520W has a pull-up resis- tor to ‘VO Bus’, and a capacitor to ‘VO COM’. This enables its Track input to rise automatically; once it is allowed to do so. In sequencing applications, the non-isolated modules are powered by the ‘VO Bus’ output. A MOSFET, internal to the PTB78520W, holds the Track voltage (and the ‘VO Seq’ output) at ground for 20 ms after the ‘VO Bus’ output is in regulation. This gives the non-isolated mod- ules time to initialize so that their outputs can rise with the ‘VO Seq’ output. Notes: 1. Auto-Track compatible modules incorporate a Track input that can take direct control of the output voltage during power-up transistions. The control relationship is on a volt-for-volt basis and is active between the 0 V and the module’s set-point voltage. When the Track input is above the set-point voltage, the module remains at its set point. Connecting the Track input of a number of such modules together allows their outputs to follow a common track control voltage during power-up. 2. When ‘+Sense’ (pin 8) is connected to the ‘VO Seq’ output (pin 10), the ‘Vo Seq’ output will be tightly regulated to the PTB78520W’s set-point voltage. In this configuration, the voltage at the ‘VO Bus’ output (pin 9) will be up to 100 mV higher. 3 The ‘VO Seq’ output cannot sink load current. This constraint does not allow the PTB78520W to coordinate a sequenced power down. 4. The slew rate for the Track input signal must be between 0.1 V/ms and 1 V/ms. Above this range the ‘VO Seq’ output may no longer accurately follow the Track input voltage. A slew rate below this range may thermally stress the converter. These slew rate limits are met whenever the Track input voltage is allowed to rise, using the internal R-C time constants at the Track input of all modules being sequenced. 5. Whenever an external voltage is used to control the Track input, the source current must be limited. A resistance value of 2.74-k Ω is recommended for this purpose. This is necessary to protect the internal transistor to the PTB78520W converter’s Track control input. This transistor holds the track control voltage at ground potential for 20 ms after the ‘VO Bus’ output is in regulation. Figure 2-1; Block Diagram of PTB78520 Auto-Track Features Smart Sense RTRK 24.9 k CTRK 1 µF Supply Supervisor 20 ms Delay Vo Bus Vo Seq (+)Sense Vo COM To feedback error amp’ Track Not Recommended for New Designs |
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