DN389 - Tracking and Sequencing Made Simple with Tiny Point-of-Load Circuit

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Tracking and Sequencing Made Simple with
Tiny Point-of-Load Circuit – Design Note 389
Scott Jackson
Introduction
Multiple-voltage electronics systems often require
complex supply voltage tracking or sequencing, which
if not met, can result in system faults or even permanent
failures in the field. The design difficulties in meeting
these requirements are often compounded in distributedpower architectures where point-of-load (POL) DC/DC
converters or linear regulators are scattered across PC
board space, sometimes on different board planes. The
problem is that power supply circuitry is often the last
circuitry to be designed into the board, and it must be
shoehorned into whatever little board real estate is left.
Often, a simple, drop-in, flexible solution is needed to
meet these requirements.
The LTC®2927 provides a simple and versatile solution in
a tiny footprint for both tracking and sequencing without
the drawbacks of series MOSFETs. Furthermore, power
EARLY VIN
3.3V
0.1µF
138k
VCC
ON
100k
RAMP
SDO
RUN/SS
IN
DC/DC
RAMPBUF
FB
16.5k
VIN
0.1µF
LTC2927
FB = 1.235V
OUT
1.8V
GND
35.7k
EARLY VIN
3.3V
Basic Operation
Each POL converter that must be tracked or sequenced can
have a single LTC2927 placed at point-of-load as shown
in Figure 1. By selecting a few resistors and a capacitor,
a supply is configured to ramp up and ramp down with
a variety of voltage profiles. The choice of resistors can
cause a slave supply to track the master signal exactly
or with a different ramp rate, voltage offset, time delay,
or combination of these.
Figure 2 shows a 4-supply tracking and sequencing
profile that highlights the flexibility of the LTC2927. A
master signal is generated by tying a capacitor from the
RAMP pin to ground or by supplying another ramping
signal to be tracked. This ramping signal can be a master
signal generated by another LTC2927 or another tracking
controller such as the LTC2923. Likewise, another supply
voltage can be used as the master signal. If an external
ramping signal is used, it can be connected directly to
the RAMP pin or to the resistive divider connected to
the TRACK pin.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TRACK
13k
supply stability and transient response remain unaffected
because the LTC2927 offsets the output voltage of the
regulator without altering the power supply control loop
dynamics.
16.5k
0.1µF
5V
VCC
ON
RAMP
VIN
LTC2927
3.3V
1V/DIV
2.5V
SDO
RUN/SS
1.8V
DC/DC
RAMPBUF
FB
887k
IN
FB = 0.8V
OUT
2.5V
TRACK
412k
GND
412k
887k
50ms/DIV
DN389 F02
DN389 F01
Figure 1. Dual Supply Tracking Application
05/06/389
Figure 2. Output Profile of a 4-Supply System Showing
Tracking, Sequencing and Ramp Rate Control
For applications that require master control of the shutdown or RUN/SS pins of the slave supplies, the LTC2927
provides an SDO output. SDO pulls low when the ON pin
is below 1.23V and the RAMP pin is below 200mV.
Negative Supply Tracking
The LTC2927 can also be used to track negative voltage
regulators. Figure 3 shows a tracking example using an
LT3462 inverting DC/DC converter to produce a –5V supply. This converter has a ground-based reference, which
allows current to be pulled from a node where RFA has
been divided. To properly pull current from the LT3462
FB network, a current mirror must be placed between the
LTC2927 and the converter. Figure 4 shows the tracking
profile of Figure 3 with a ramp rate of 100V/s. VMASTER
is positive, but the inverse is shown for clarity. The –5V
slave does not pull all the way up to 0V at VMASTER = 0V
because the ground referenced current mirror cannot
pull its output all the way to ground. If the converter has
an FB reference voltage greater than 0V or if a negative
supply is available for the current mirror, the offset can
be removed. Figure 5 shows the resulting waveform.
Conclusion
The LTC2927 simplifies power supply tracking and
sequencing by offering superior performance in a tiny
point-of-load footprint. Only a few resistors are needed
to configure simple or complex supply behaviors. Series
MOSFETs are eliminated along with their parasitic voltage drops and power consumption. The LTC2927 offers
all of these features in tiny 8-lead ThinSOTTM and 8-lead
(3mm × 2mm) DFN package.
1µF
22µH
EARLY
VIN
6V
0.1µF
RONB
487k
22µH
1µF
VCC
ON
RAMP
RONA
100k
MASTER
CRAMP
0.1µF
LTC2927
SW
VREF
D
SDREF = 1.25V
DMMT5551
RAMPBUF
RTB1
137k
LT3462
FB
RFB
274k
22pF
RFA/2
34k
TRACK
RTA1
26.1k
RFA/2
34k
–5V
100mA
10µF SLAVE
FB
GND
DN389 F03
Figure 3. Supply Tracking of a GND Referenced Negative Regulator
0V
SLAVE
0V
–VMASTER
1V/DIV
1V/DIV
–5V SLAVE
–VMASTER
10ms/DIV
DN389 F04
Figure 4. Tracking Profile of the Negative Regulator
Application in Figure 3
–5V SLAVE
–VMASTER
10ms/DIV
DN389 F05
Figure 5. Tracking Profile of the Negative Regulator Application Without the Current Mirror Pull-Down Limitation
Data Sheet Download
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call (408) 432-1900, Ext. 2452
www.linear.com
Linear Technology Corporation
dn389f LT/TP 0506 305K • PRINTED IN THE USA
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