12. Output Ripple Attenuator Module (MicroRAM)

12. Output Ripple Attenuator Module (MicroRAM)
Design Guide & Applications Manual
For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies
RSENSE
5.1
+IN
+OUT
PC
+S
22 m F
DC-DC
Converter
+OUT
+IN
RHR
SC
SC
Vref
CTRAN
PR
–S
–IN
–OUT
CTRAN*
LOAD
CHR*
–OUT
–IN
*Optional Component
Figure 12–1a — Typical configuration using remote sense
20 kΩ
IRML6401
PC
PR
–IN
DC-DC
Converter
RSC
SC
SC
CTRAN
–OUT
+OUT
+IN
+OUT
+IN
RHR
μRAM
–IN
CTRAN*
Vref
LOAD
CHR*
1 µF
–OUT
*Optional Component
Figure 12–1b — Typical configuration using SC control (Optional CHR, 25 µF maximum in SC configuration.)
FUNCTIONAL DESCRIPTION
The MicroRAM has an internal passive filter, (Figure 12–2)
that effectively attenuates ripple in the 50 kHz to 1 MHz
range. An active filter provides attenuation from low
frequency up to the 1 MHz range. The user must set the
headroom voltage of the active block with the external
RHR resistor to optimize performance. The MicroRAM
must be connected as shown in Figures 12–1a or 12–1b
depending on the load-sensing method. The transient
load current performance can be increased by the addition
of optional CTRAN capacitance to the CTRAN pin. The lowfrequency ripple attenuation can be increased by addition
of optional CHR capacitance to the VREF pin as shown in
Figures 12–3a and 12–3b.
Transient load current is supplied by the internal CTRAN
capacitance, plus optional external capacitance, during the
time it takes the converter loop to respond to the increase
in load. The MicroRAM’s active loop responds in roughly
one microsecond to output voltage perturbations. There
are limitations to the magnitude and the rate of change
of the transient current that the MicroRAM can sustain
while the converter responds. See Figures 12–8 through
Maxi, Mini, Micro Design Guide
Page 56 of 88
12–16 for examples of dynamic performance. A larger
headroom voltage setting will provide increased transient
performance, ripple attenuation, and power dissipation
while reducing overall efficiency. (Figures 12–4a, 12–4b,
12–4c, and 12–4d)
The active loop senses the output current and reduces the
headroom voltage in a linear fashion to approximate
constant power dissipation of MicroRAM with increasing
loads. (Figures 12–7, 12–8 and 12–9) The headroom
setting can be reduced to decrease power dissipation
where the transient requirement is low and efficient ripple
attenuation is the primary performance concern.
The active dynamic headroom range is limited on the low
end by the initial headroom setting and the maximum
expected load. If the maximum load in the application is
10 A, for example, the 1 A headroom can be set
75 mV lower to conserve power and still have active
headroom at the maximum load current of 10 A. The
high end or maximum headroom range is limited by the
internal ORing diode function.
Rev 4.9
Apps. Eng. 800 927.9474
vicorpower.com
800 735.6200
12. Output Ripple Attenuator Module (MicroRAM)
Design Guide & Applications Manual
For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies
The SC or trim-up function can be used when remote
sensing is not available on the source converter or is not
desirable. It is specifically designed for converters with a
1.23 V reference and a 1 kΩ input impedance like Vicor
Maxi, Mini, Micro converters. In comparison to remote
sensing, the SC configuration will have an error in the
load voltage versus load current. It will be proportional to
the output current and the resistance of the load path
from the output of the MicroRAM to the load.
Load capacitance can affect the overall phase margin of
the MicroRAM active loop as well as the phase margin
of the converter loop. The distributed variables such as
inductance of the load path, the capacitor type and
value as well as its ESR and ESL also affect transient capability at the load. The following guidelines should
be considered when point-of-load capacitance is used
with the MicroRAM in order to maintain a minimum of
30 degrees of phase margin.
The ORing feature prevents current flowing from the
output of the MicroRAM back through its input terminal
in a redundant system configuration in the event that a
converter output fails. When the converter output
supplying the MicroRAM droops below the ORed output
voltage potential of the redundant system, the input of
the MicroRAM is isolated from it’s output. Less than
50 mA will flow out of the input terminal of the MicroRAM
over the full range of input voltage under this condition.
1. Using ceramic load capacitance with <1 mΩ
ESR and <1 nH ESL:
a. 20 µF to 200 µF requires 20 nH of trace / wire
load path inductance
b. 200 µF to 1,000 µF requires 60 nH of trace / wire
load path inductance
2. For the case where load capacitance is connected
directly to the output of the MicroRAM, i.e. no trace
inductance, and the ESR is >1 mΩ:
a. 20 µF to 200 µF load capacitance needs an ESL
of >50 nH
Passive
Block
+In
Active
Block
SC
Vref
SC
Control
CTRAN
b. 200 µF to 1,000 µF load capacitance needs an
ESL of >5 nH
+Out
3. Adding low ESR capacitance directly at the output
terminals of MicroRAM is not recommended and may
cause stability problems.
–Out
–In
4. In practice, the distributed board or wire inductance
at a load or on a load board will be sufficient to
isolate the output of the MicroRAM from any load
capacitance and minimize any appreciable effect on
phase margin.
Figure 12-2 — MicroRAM block diagram
Ripple Attenuation @ 5 V (Room Temp.)
20.00
0.00
0.00
Gain (dB)
Gain (dB)
Ripple Attenuation @ 28 V (Room Temp.)
20.00
-20.00
-40.00
-40.00
-60.00
-60.00
-80.00
10
-20.00
100
1,000
10,000
100,000
-80.00
10
1,000,000 10,000,000
100
1,000
Freq. (Hz)
10 A, 100 uF Vref
Page 57 of 88
100,000
1,000,000
10,000,000
Freq. (Hz)
10 A, No Vref Cap
Figure 12–3a — The small signal attenuation performance as
measured on a network analyzer with a typical module at 28 V and
10 A output. The low frequency attenuation can be enhanced by
connecting a 100 µF capacitor, CHR, to the VREF pin as shown in
Figures 12–1 and 12–2.
Maxi, Mini, Micro Design Guide
10,000
10 A, 100 uF Vref
10 A, No Vref Cap
Figure 12–3b — The small signal attenuation performance as
measured on a network analyzer with a typical module at 5 V and
10 A. The low frequency attenuation can be enhanced by connecting a 100 µF capacitor, CHR, to the VREF pin as shown in Figures
12–1 and 12–2.
Rev 4.9
Apps. Eng. 800 927.9474
vicorpower.com
800 735.6200
12. Output Ripple Attenuator Module (MicroRAM)
Design Guide & Applications Manual
For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies
-0
Rhr = 28 k (Vheadroom = 90 mV)
27 k (100 mV)
26 k (110 mV)
25 k (122 mV)
24 k (135 mV)
23 k (150 mV)
22 k (160 mV)
Vout = 3 V load = 20 A
100 degrees baseplate temperature
-25
-50
17 k (260 mV)
18 k (240 mV)
19 k (217 mV)
20 k (197 mV)
21 k (180 mV)
-75
10 Hz
100 Hz
1.0 KHz
... DB(V(VOUT))
10 KHz
100 KHz
1.0 MHz
Frequency
Figure 12–4a — Graph of simulated results demonstrating the tradeoff of attenuation vs. headroom setting at 20 A and a equivalent 100°C
baseplate temperature at 3 V.
-0
Rhr = 260 k (Vheadroom = 90 mV)
250 k (100 mV)
240 k (110 mV)
230 k (122 mV)
220 k (135 mV)
210 k (150 mV)
200 k (160 mV)
Vout = 28 V load = 20 A
100 degrees baseplate temperature
-25
-50
150 k (260 mV)
160 k (240 mV)
170 k (217 mV)
180 k (197 mV)
190 k (180 mV)
-75
10 Hz
100 Hz
1.0 KHz
... DB(V(VOUT))
10 KHz
100 KHz
1.0 MHz
Frequency
Figure 12–4b — Graph of simulated results demonstrating the tradeoff of attenuation vs. headroom setting at 20 A and a equivalent 100°C
baseplate temperature at 28 V.
-10
-10
Rhr = 260 k
dB
-30
-20
100 khz 3 V
500 khz 3 V
1 Mhz 3 V
Rhr=28 k
27 k
-30
26 k
25 k
-40
24 k
23 k
-50
22 k
21 k
20 k
18 k
17 k
4.0
4.5
5.0
5.5
210 k
200 k
190 k
180 k
170 k
160 k
150 k
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Watts
Figure 12–4c — MicroRAM attenuation vs. power dissipation at 3 V, 20 A
Page 58 of 88
220 k
-40
-70
6.0
Watts
Maxi, Mini, Micro Design Guide
230 k
-60
-70
3.5
100 khz 28 V
500 khz 28 V
1 Mhz 28 V
-50
19 k
-60
3.0
250 k
240 k
dB
-20
Figure 12–4d — MicroRAM attenuation vs. power dissipation at 28 V, 20 A
Rev 4.9
Apps. Eng. 800 927.9474
vicorpower.com
800 735.6200
12. Output Ripple Attenuator Module (MicroRAM)
Design Guide & Applications Manual
For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies
450 mV
VOUT=3 V
Vheadroom
400 mV
Rhr=16k
300 mV
17k
18k
19k
20k
21k
200 mV
1A
2A
4A
6A
8A
10A
12A
14A
16A
18A
20A
lload
Figure 12–5 — Headroom vs. load current at 3 V output
450 mV
VOUT = 15 V
Vheadroom
400 mV
Rhr = 80 k
300 mV
85 k
90 k
95 k
100 k
105 k
200 mV
1 A 2 A
4 A
6 A
8 A
10 A
12 A
14 A
16 A
18 A
20 A
lload
Figure 12–6 — Headroom vs. load current at 15 V output
450 mV
VOUT=28 V
Vheadroom
400 mV
Rhr=150k
300 mV
160k
170k
180k
190k
200k
200 mV
1 A 2 A
4 A
6 A
8 A
10 A
12 A
14 A
16 A
18 A
lload
Figure 12–7 — Headroom vs. load current at 28 V output
Maxi, Mini, Micro Design Guide
Page 59 of 88
Rev 4.9
Apps. Eng. 800 927.9474
vicorpower.com
800 735.6200
20 A
12. Output Ripple Attenuator Module (MicroRAM)
Design Guide & Applications Manual
For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies
Figure 12–8 — V375A28C600B and µRAM: Input and output
ripple @ 50% (10 A) load CH1 = Vi; CH2 = Vo; Vi – Vo = 332 mV;
RHR = 178 k
Figure 12–9 — V375A28C600B and µRAM; Input and output dynamic
response no added CTRAN; 20% of 20 A rating load step of 4 A
(10 A – 14 A); RHR = 178 k (Configured as in Figures 14–1a and 14–1b)
Figure 12–10 — V375A28C600B and µRAM; Input and output
dynamic response CTRAN = 820 µF Electrolytic; 33% of load step of
6.5 A (10 A–16.5 A); RHR = 178 k (Configured as in Figures 14–1a
and 14–1b)
Figure 12–11 — V375B12C250B and µRAM; Input and output ripple
@50% (10 A) load CH1 = Vi; CH2 = Vo; Vi – Vo = 305 mV; RHR = 80 k
(Configured as in Figures 14–1a and 14–1b)
Figure 12–12 — V300B12C250B and µRAM; Input and output
dynamic response no added CTRAN; 18% of 20 A rating load step of
3.5 A (10 A – 13.5 A); RHR = 80 k (Configured as in Figures 14–1a
and 14–1b)
Figure 12–13 — V300B12C250B and µRAM; Input and output
dynamic response CTRAN = 820 µF Electrolytic; 30% of load step
of 6 A (10 A – 16 A); RHR = 80 k (Configured as in Figures14–1a
and 14–1b)
Maxi, Mini, Micro Design Guide
Page 60 of 88
Rev 4.9
Apps. Eng. 800 927.9474
vicorpower.com
800 735.6200
12. Output Ripple Attenuator Module (MicroRAM)
Design Guide & Applications Manual
For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies
Figure 12–14 — V48C5C100B and µRAM; Input and output ripple
@ 50% (10 A) load CH1 = Vi; CH2 = Vo; Vi – Vo = 327 mV;
RHR = 31 k (Configured as in Figures 12–1a and 12–1b)
Figure 12–15 — V48C5C100B and µRAM; Input and output
dynamic response no added CTRAN; 23% of 20 A rating load step
of 4.5 A (10 A – 14.5 A); RHR = 31 k (Configured as in Figures 12–1a
and 12–1b)
Figure 12–16 — V48C5C100B and µRAM; Input and output
dynamic response CTRAN = 820 µF Electrolytic; 35% of load step
of 7 A (10 A – 17 A); RHR = 31 k (Configured as in Figures 12–1a
and 12–1b)
NOTES: The measurements in Figures 12–8 through 12–16 were taken with a µRAM2C21 and standard scope probes
set at 20 MHz bandwidth scope setting.
The criteria for transient current capability was as follows: The transient load current step was incremented
from 10 A to the peak value indicated, then stepped back to 10 A until the resulting output peak to peak
measured ~ 40 mV.
Maxi, Mini, Micro Design Guide
Page 61 of 88
Rev 4.9
Apps. Eng. 800 927.9474
vicorpower.com
800 735.6200