DEMO CIRCUIT 817A
QUICK START GUIDE
LTC2927
Single Power Supply
Tracking Controller
DESCRIPTION
Demonstration circuit DC817A is intended for
evaluating the performance of the LTC2927 Single
Power Supply Tracking Controller. The board
contains two LTC2927s tied to a common ramping
signal, providing tracking and sequencing for two
power supplies. As assembled, DC817A operates
directly with two DC222 buck outputs.
Design files for this circuit board are available.
Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT and PowerPath are trademarks of Linear Technology Corporation.
PERFORMANCE SUMMARY
Specification s are at T
A
= 25°
C
SYMBOL
V
CC
V
CC(UVLO)
•V
CC(UVLO)
V
ON(TH)
•V
ON(HYST)
V
TRACK
PARAMETER
Input Supply Voltage Range
Supply Undervoltage Lockout
Supply Undervoltage Lockout Hysteresis
ON Pin Threshold Voltage
ON Pin Hysteresis
TRACK Pin Voltage
CONDITIONS
V
CC RISING
V
ON RISING
MIN
2.6
2.2
1.210
30
0.77
0.77
1.5
TYP
2.5
25
1.230
75
0.8
0.8
2.0
MAX
5.5
2.7
1.250
150
0.82
0.82
2.3
UNITS
V
V
mV
V
mV
V
V
V
V
FB(CLAMP)
FB Pin Clamp Voltage
V
TRACK
= -10µA
V
TRACK
= -1mA
1µA•I
FB
•1mA
OPERATING PRINCIPLES
The LTC2927 controls the output voltage of a
power supply by introducing a small current into the
supply’s feedback node. When this current is at its
maximum, the supply’s output is forced to zero;
decreasing this current to zero allows the supply’s
output to rise to its normal, regulated output.
The RAMP pin is driven by an internally generated
constant current source, which when loaded with
capacitor, produces a linear voltage ramp. By
varying the injected current under the control of this
“master” ramping signal, the slave supply output is
made to follow the master ramp in a predictable
fashion.
Any slave power supply, which has an accessible
inverting feedback node of less than 1.5V, and a
feedback divider current in the range of 10µA to
1mA, may be controlled with the LTC2927.
Tracking behavior is configured using two resistors,
RTA and RTB. A valid Vcc supply for the LTC2927
must be present before the tracked supply is
enabled or powered up.
Boost converters and certain linear regulators are
generally incompatible with the LTC2927 control
technique.
DC817A includes two LTC2927 controllers driven
by a common ramp signal. The first controller
1
generates the ramp signal and monitors Vcc with
its ON pin. The second controller’s RAMP pin is
driven by REMPBUF of the first; its ON pin is
defeated by being tied to V
CC
.
To reconfigure DC817A for use with other supplies,
five components may be changed: C3 to control
the master ramp rate, and RTA1, RTB1, RTA2, and
RTB2 to control the relative behavior of the two
supplies. Hold-off time, ramp rate, and offset are all
defined by the resistors. For details on calculating
these component values, consult the LTC2927 data
sheet.
QUICK START PROCEDURE
As a design example, DC817A is populated for
operation with two DC222 slave supplies, using the
LT1735 Buck Regulator, which are configured for
3.3V and 2.5V output. The first LTC2927 controls
the 3.3V supply, while the second controls the 2.5V
supply.
Connect the slave supplies as shown in Figure 1,
attaching both the RUN/SS and VOSENSE lines to
the DC817A. Next apply 5V to all boards; the
outputs will begin tracking up once the ON pin of
U1 detects a valid 5V input.
The tracking waveforms are shown in Figure 2 and
Figure 3. Also shown in Figure 2 and Figure 3 is an
optional “enable “ waveform, which is simply a
switch closure to ground on U1’s ON pin (see
Figure 1 for location of this pad). This may be
driven by an open collector, such as an enable
signal from a microcontroller or separate supply
monitor.
.
Figure 1. Test Setup
2
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