Texas Instruments | Understanding frequency variation in the DCS-Control(TM) topology | Application notes | Texas Instruments Understanding frequency variation in the DCS-Control(TM) topology Application notes

Texas Instruments Understanding frequency variation in the DCS-Control(TM) topology Application notes
Analog Applications Journal
Automotive
Understanding frequency variation in the
DCS-Control™ topology
By Chris Glaser
Applications Engineer
Introduction
DCS-Control topology used in a typical automotive infotainment device.[1, 2]
As explained in Reference 1, the timer (tON_MIN) is
responsible for providing a controlled switching frequency
by adjusting the on-time based on VIN and VOUT through
Equation 1.
A common requirement in automotive, communications
equipment and industrial markets is to avoid interfering
with specific frequency ranges, such as the AM radio band,
to minimize disturbances for sensitive electronics, such as
sensors. One example for reducing interference from
power supplies is setting their switching frequency above
the sensitive frequencies to keep switching noise at a
higher frequency.
If the frequency is set below sensitive frequencies, then
higher-frequency harmonics would be in band and possibly
cause interference. Most modern power supplies do not
use an actual oscillator to set their switching frequency, as
in traditional voltage- or current-mode control. Instead,
either the on-time or off-time is controlled, which then
provides a relatively constant operating frequency.
DCS-Control™ topology is an example of a topology
that is on-time based, which efficiently provides the lownoise and fast-transient response needed in many applications. While the switching frequency of this topology does
vary, this variation is understood, controlled, and usually
sufficient for frequency-sensitive applications such as
automotive, communications equipment, test and
measurement, and factory automation.
tON =
VOUT
× 400 ns
VIN
(1)
The 400-ns value sets the ideal switching frequency to
2.5 MHz when the DCS-Control device is operating with
the on-time set by the timer. However, due to circuit
losses, propagation delays, and in some specific application conditions, operation does not always follow the
on-time set by the timer. As a result, the frequency varies.
The reasons for this variation are grouped together based
on the duty cycle, ideally VOUT/VIN, at which the device
operates.
Measured data explains the principles behind the
DCS-Control topology’s frequency variation. To better
explain the concepts, the TPS62130 (catalog version) was
chosen and it offers two switching frequencies: 2.5 MHz
and 1.25 MHz. The 2.5-MHz data exactly matches the
TPS62130A-Q1 data because both converters offer the
2.5-MHz setting. All data was taken on the evaluation
module with a 2.2-µH inductor and two 22-µF output
capacitors (to overcome the DC bias effect).[3]
Application example
Figure 1 shows the basic block diagram of the
Figure 1. Block diagram of the DCS-Control™
topology in the TPS62130A-Q1 converter
Control
VOS
FB
Direct Control
and
Compensation
–
+
Ramp
Comparator
Timer (t ON_min)
Error
Amplifier
VREF
0.8 V
Texas Instruments
Gate
Driver
DCS-Control TM Topology
4
AAJ 4Q 2015
Analog Applications Journal
Automotive
Moderate duty cycles
Figure 2. TPS62130 with a 5-V output
In the typical application of converting the 12-V car
battery to 5 V for universal serial bus (USB) ports, the
required duty cycle is not extremely high or low.
Frequency variation in this case is very low because the
on- and off-times are not at their extremes. Figure 2
shows the measured switching frequency, on-time, and
off-time with a 5-V output voltage, two frequency settings,
and two different load currents. A moderate duty cycle
refers to those input voltages above 9 V for the 2.5-MHz
setting and above 7 V for the 1.25-MHz setting.
Figure 2b shows the reason behind the low levels of
frequency variation. The on-time matches very well to the
ideal on-time set by the timer and to Equation 1 for both
loads and frequency settings. The reasons for the small
frequency variation with moderate duty cycles are: overcoming losses and propagation delays.
In Figure 2a, the frequency increases with heavier loads
due to losses. Higher loads require slightly higher duty
cycles to overcome resistive losses in the circuit. Since the
on-times are the same for both the 1-A load and 3-A load,
the off-time is decreased to achieve the higher duty cycle
(Figure 2c). The same on-time and a shorter off-time
results in a shorter period and higher frequency.
Also, the frequency decreases slightly with increasing
input voltage. Because the on-time decreases with increasing input voltage, fixed propagation delays in the device
have a more significant effect on the achieved on-time for
smaller on-time values. The timer sets the on-time to
achieve a certain frequency, but its output signal goes
through the control and gate driver (shown in Figure 1)
before reaching the power transistors. This path takes
some finite amount of time. For example, if a 200-ns
on-time is desired and the propagation delay is 20 ns, the
actual on-time is 220 ns, which is 10% higher than desired.
But, if the input voltage increases and the desired on-time
reduces to 100 ns, the same 20-ns delay produces a 20%
increase in the actual on-time. This effect is further
pronounced for low duty cycles.
Switching Frequency (MHz)
3.0
2.5
2.5 MHz_3 A
2.5 MHz_1 A
2.0
1.5
1.25 MHz_3 A
1.0
1.25 MHz_1 A
0.5
0
5
6
7
8
9
10 11 12 13 14 15 16 17
Input Voltage (V)
(a) Switching frequency
1000
1.25 MHz_3 A_On_Time
900
1.25 MHz_1 A_On_Time
On-Time (ns)
800
1.25 MHz_Ideal
2.5 MHz_3 A_On_Time
700
2.5 MHz_1 A_On_Time
600
2.5 MHz_Ideal
500
400
300
200
100
5
6
7
8
9 10 11 12 13 14 15 16 17
Input Voltage (V)
(b) On-time
600
High duty cycles
550
While a car battery nominally operates at ~12 V, transients
from high-current loads, such as starting the engine, can
reduce the battery voltage. To the power supply this
appears as a line transient, which means more advanced
regulation is required in some applications. As long as the
input voltage does not decrease below the level of the
output voltage, the DCS-Control topology maintains
output regulation during such line and load transients.
1.25 MHz_1 A_Off_Time
500
1.25 MHz_3 A_Off_Time
Off-Time (ns)
450
400
350
2.5 MHz_1 A_Off_Time
300
2.5 MHz_3 A_Off_Time
250
200
150
100
5
6
7
8
9
10 11 12 13 14 15 16 17
Input Voltage (V)
(c) Off-time
Texas Instruments
5
AAJ 4Q 2015
Analog Applications Journal
Automotive
Figure 3 shows measured data for a 3.3-V output. When
the input voltage of the converter drops, the duty cycle
increases. At high duty cycles, the switching frequency
decreases due to losses and a minimum off-time. High
duty cycles refers to the left-most portion of Figures 2a
and 3a where the switching frequency decreases from its
nominal value towards zero.
High duty cycles demonstrate a minimum off-time in the
topology. Since high duty cycles occur at a lower input
voltage and higher output voltage, the energy stored in the
inductor during the on-time is lower. This outcome is
because there is much less voltage across the inductor. To
maximize efficiency, a minimum off-time is included to
ensure that sufficient energy is delivered to the output.
This is especially helpful in power-save mode, in which a
certain amount of energy is delivered so the output stays
higher for a longer time. This results in a gap between
switching pulses and higher efficiency. From Figure 3c,
once the minimum off-time is reached (around a 6-V input
voltage for the 2.5-MHz setting), the on-time begins to rise
from ideal in order to achieve the required increase in
duty cycle that corresponds to the lower input voltage.
Figure 2c and Figure 3c show the 120-ns approximate
value of the minimum off-time.
Furthermore, the minimum off-time is quickly reached
at high duty cycles because the input voltage value is
lower as well. At input voltages below 6 V, the resistance
of the high-side MOSFET (RDS(on)) inside the DCS-Control
device increases, thus creating higher losses and a greater
required extension of the duty cycle. For example, 3-A
loads show longer on-times than 1-A loads at lower input
voltages.
Figure 3. TPS62130 with a 3.3-V output
Switching Frequency (MHz)
3.0
2.5
2.5 MHz_3 A
2.5 MHz_1 A
2.0
1.5
1.0
1.25 MHz_3 A
1.25 MHz_1 A
0.5
0
4
5
6
7
8
9 10 11 12 13 14 15 16 17
Input Voltage (V)
(a) Switching frequency
1000
900
1.25 MHz_3 A_On_Time
800
1.25 MHz_1 A_On_Time
1.25 MHz_Ideal
On-Time (ns)
700
2.5 MHz_3 A_On_Time
600
2.5 MHz_1 A_On_Time
500
2.5 MHz_Ideal
400
300
200
Low duty cycles
100
Low duty cycles occur with lower output voltages, such as
1 V and 1.8 V. The relatively high 12-V input voltage
requires duty cycles of sometimes less than 10%. With
respect to the desired 400-ns period, this requires
on-times near and even below 40 ns. Such small on-times
are challenging for any converter to achieve, or are actually impossible due to absolute minimum on-times. The
TPS62130 data sheet notes a typical 80-ns absolute
minimum on-time that occurs in these cases. This is the
primary source of frequency variation at low duty cycles.
Fixed propagation delays added to small on-times are
another source of variation, as explained before. Figure 4
shows measured data for a 1.8-V output voltage.
0
4
5
6
7
8
9 10 11 12 13 14 15 16 17
Input Voltage (V)
(b) On-time
700
1.25 MHz_1 A_Off_Time
Off-Time (ns)
600
1.25 MHz_3 A_Off_Time
500
400
2.5 MHz_1 A_Off_Time
2.5 MHz_3 A_Off_Time
300
200
100
4
5
6
7
8
9 10 11 12 13 14 15 16 17
Input Voltage (V)
(c) Off-time
Texas Instruments
6
AAJ 4Q 2015
Analog Applications Journal
Automotive
The 2.5-MHz curves in Figure 4b clearly show a minimum
on-time in the 80-ns range. This sets an upper boundary
on the achievable switching frequency. The 1.25-MHz
curves show good frequency variation similar to Figures 2a
and 3a. Due to smaller on-times with this 1.8-V output,
fixed propagation delays cause a sharper downward
frequency shift versus higher output voltages, which result
in a lower frequency.
Additionally, the bumpiness seen in the 2.5-MHz curves
(Figure 4a) shows a third impact to the on-time: the
comparator. During a transient, the comparator extends
the on-time past the output of the timer to deliver more
energy to the output to make the output voltage recover
faster. This is a key aspect of a hysteretic converter and
explains the fast transient response of the DCS-Control
topology.
While the 80-ns minimum on-time and the output of the
timer do not change much over the input voltage range,
the output signal does change due to the changing ripple on
the inductor current. There is increased ripple with higher
input voltages. Having more ripple across the equivalent
series resistance (ESR) and equivalent series inductance
(ESL) in the output capacitors creates more signal for the
comparator on which to react, making the system faster.
Between 12 and 13 V, there is enough signal and the
comparator no longer controls the on-time. The minimum
on-timer controls it. Thus, higher frequency is achieved
above this input voltage.
One solution to the lower frequency is a two-stage
conversion of the 12 V to the load. A two-stage conversion
(via 5 V, for example) to very-low output voltages achieves
a higher frequency in both stages because of the more
moderate on-times of each stage.
Finally, the lower switching frequency that occurs with
lower output voltages will increase the inductor current
ripple, but this ripple is already lowered because of the
low output voltage (Equation 2). Lower output voltages
have less current ripple to begin with. When following the
datasheet recommendations for inductance and switching
frequency, this lower switching frequency does not limit
the output current below the 3-A device rating.
∆I L(max) = VOUT
V


1 − OUT 
VIN(max)
×


 L(min) × fSW 
Figure 4. TPS62130 with a 1.8-V output
Switching Frequency (MHz)
3.0
2.5 MHz_1 A
2.0
1.5
1.0
1.25 MHz_3 A
1.25 MHz_1 A
0.5
0
3
4
5
6
7
8 9 10 11 12 13 14 15 16 17
Input Voltage (V)
(a) Switching frequency
400
1.25 MHz_3 A_On_Time
350
1.25 MHz_1 A_On_Time
1.25 MHz_Ideal
On-Time (ns)
300
2.5 MHz_3 A_On_Time
2.5 MHz_1 A_On_Time
250
2.5 MHz_Ideal
200
150
100
50
0
3
4
5
6
7
8 9 10 11 12 13 14 15 16 17
Input Voltage (V)
(b) On-time
current capability. High-frequency operation is maintained
for the common applications of USB ports and system rails
with higher voltages.
References
1. Chris Glaser, “High-efficiency, low-ripple DCS-Control
offers seamless pulse-width modulation (PWM)/power-save
transitions,” TI Analog Applications Journal, 3Q 2013
(SLYT531)
2. Datasheet, “TPS6213xA-Q1 3V to 17-V 3A Step-Down
Converter with DCS-Control™,” Texas Instruments,
May 2014
3. TPS62130EVM-505, “Evaluation Module for TPS62130 a
3-A, synchronous, step-down converter in a 3x3-mm,
16-pin QFN,” Texas Instruments
(2)
Conclusion
The switching frequency of the DCS-Control topology and
other non-oscillator-based control topologies vary with
changes in the application conditions. Depending on the
duty cycle, the on-time and the frequency are affected by
losses, the minimum off-time, the absolute minimum
on-time, propagation delays, or the comparator. This
behavior is understood and expected, and output voltage
regulation is maintained. The lower operating frequency
provides higher efficiency with no reduction in output
Texas Instruments
2.5 MHz_3 A
2.5
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