Texas Instruments TPS51116 Memory Power Solution, Synchronous Buck Controller Evaluation Module TPS51116EVM-001 TPS51116EVM-001 Datenbogen
Produktcode
TPS51116EVM-001
Appendix A
A.1
D-CAP Mode and Transient Response
Appendix A
www.ti.com
TI’s adaptive constant ON-time semi-hysteretic control scheme or D-CAP technology uses V
IN
and V
OUT
to
determine an ON-time. After this ON-time, the controller waits a minimum off time, then detects a valley in
the output voltage, and turns back on after the output voltage has dropped below a threshold voltage. By
using a variant of voltage feed-forward, the constant ON-time is adapted to maintain a near constant
switching frequency during steady state operation, but allows for the advantages of hysteretic control
during a load transient.
the output voltage, and turns back on after the output voltage has dropped below a threshold voltage. By
using a variant of voltage feed-forward, the constant ON-time is adapted to maintain a near constant
switching frequency during steady state operation, but allows for the advantages of hysteretic control
during a load transient.
Turning a positive load transient, the output voltage drops below the threshold and the OFF-time
comparator reacts as soon as the minimum OFF-time has elapsed, increasing the switching frequency
and immediately driving the output to its maximum duty cycle. Once the output has charged above the
valley threshold, the OFF-time is again set by the decay of the output voltage. Since there is no
compensation capacitor to charge or discharge, the TPS51116 is able to immediately react to load
transient without recovery ringing. (See
comparator reacts as soon as the minimum OFF-time has elapsed, increasing the switching frequency
and immediately driving the output to its maximum duty cycle. Once the output has charged above the
valley threshold, the OFF-time is again set by the decay of the output voltage. Since there is no
compensation capacitor to charge or discharge, the TPS51116 is able to immediately react to load
transient without recovery ringing. (See
and
In
, a 0 A to 8 A load transient with the output voltage and the inductor ripple current. On the
positive transient edge, the TPS51116 has reacted to the 8-A transient load and returned to steady state
operation within 2 ms. Despite the transient, there has been less than 50 mV of overshoot and no ringing
on the output. On the negative transient edge (as shown in
operation within 2 ms. Despite the transient, there has been less than 50 mV of overshoot and no ringing
on the output. On the negative transient edge (as shown in
), the synchronous MOSFET has been
turned on within one switching cycle and held on until the output voltage recovers from the output
transient, at which point it immediately recovers its steady state operation since the ON-time is fixed by
the input to output voltage ratio and is not a function of the voltage across a compensation capacitor. This
allows the TPS51116 to react to sever load transients extremely quickly, allowing the use of less bulk
capacitance while maintaining tighter control of the sensitive output voltage.
transient, at which point it immediately recovers its steady state operation since the ON-time is fixed by
the input to output voltage ratio and is not a function of the voltage across a compensation capacitor. This
allows the TPS51116 to react to sever load transients extremely quickly, allowing the use of less bulk
capacitance while maintaining tighter control of the sensitive output voltage.
It is important to note that the D-CAP mode depends on the output voltage relating directly to the inductor
current. This dictates that at the switching frequency, approximately 400 kHz during steady state
operation, the output capacitors should approximate a resistive load. Ceramic and ultra-low ESR
capacitors can impose a significant phase shift and interfere with D-CAP mode operation. Thus, it is not
recommended that one use ceramic capacitors when operating in D-CAP mode.
current. This dictates that at the switching frequency, approximately 400 kHz during steady state
operation, the output capacitors should approximate a resistive load. Ceramic and ultra-low ESR
capacitors can impose a significant phase shift and interfere with D-CAP mode operation. Thus, it is not
recommended that one use ceramic capacitors when operating in D-CAP mode.
18
SLUU202A – SEPTEMBER 2004 – Revised November 2008