TPS51216 Datasheet, PDF (15/30 Pages) Texas Instruments – Complete DDR2, DDR3 and DDR3L Memory Power Solution Synchronous Buck Controller, 2-A LDO, Buffered Reference

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APPLICATION INFORMATION
TPS51216
SLUSAB9 – NOVEMBER 2010
VDDQ Switch Mode Power Supply Control
TPS51216 supports D-CAP™ mode which does not require complex external compensation networks and is
suitable for designs with small external components counts. The D-CAP™ mode provides fast transient response
with appropriate amount of equivalent series resistance (ESR) on the output capacitors. An adaptive on-time
control scheme is used to achieve pseudo-constant frequency. The TPS51216 adjusts the on-time (t_ON) to be
inversely proportional to the input voltage (V_IN) and proportional to the output voltage (V_DDQ). This makes a
switching frequency fairy constant over the variation of input voltage at the steady state condition.
VREF and REFIN, VDDQ Output Voltage
The part provides a 1.8-V, ±0.8% accurate, voltage reference from VREF. This output has a 300-mA (max)
current capability to drive the REFIN input voltage through a voltage divider circuit. A capacitor with a value of
0.1-mF or larger should be attached close to the VREF terminal.
The VDDQ switch-mode power supply (SMPS) output voltage is defined by REFIN voltage, within the range
between 0.7 V and 1.8 V, programmed by the resister-divider connected between VREF and GND. (See External
Components Selection section.) A few nano farads of capacitance from REFIN to GND is recommended for
stable operation.
Soft-Start and Powergood
TPS51216 provides integrated VDDQ soft-start functions to suppress in-rush current at start-up. The soft-start is
achieved by controlling internal reference voltage ramping up. Figure 29 shows the start-up waveforms. The
switching regulator waits for 400ms after S5 assertion. The MODE pin voltage is read in this period. A typical
VDDQ ramp up duration is 700ms.
TPS51216 has a powergood open-drain output that indicates the VDDQ voltage is within the target range. The
target voltage window and transition delay times of the PGOOD comparator are ±8% (typ) and 1-ms delay for
assertion (low to high), and ±16% (typ) and 330-ns delay for de-assertion (high to low) during running. The
PGOOD comparator is enabled 1.1 ms after VREF is raised high and the start-up delay is 2.5 ms. Note that the
time constant which is composed of the REFIN capacitor and a resistor divider needs to be short enough to
reach the target value before PGOOD comparator enabled.
S5
VREF
VDDQ
PGOOD
400 ms
700 ms
1.4 ms
Figure 29. Typical Start-up Waveforms
UDG-10137
Copyright © 2010, Texas Instruments Incorporated
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