Datasheet

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Establishing Tracking and Designing a Tracking Control Loop
+
COMP
FB
1
2
20
4
690 mV
+
3
TRKOUT
To PWM
A
A
VO
TRKIN
R
BIAS
UDG−04145
R
2
R
1
C
2
C
1
R
3
C
3
C
5
C
4
R
4
V
OUT
V
IN
R
5
R
6
D
1
R
4
t
ƪ
V
HTRKOUT(min)
* V
DIODE
* V
FB
ƫ
V
FB
R
1
R
BIAS
R
1
) R
BIAS
W
(42)
dV
OUT
dV
TRKOUT
+*
R
1
R
4
(dimensionless)
(43)
TPS40100
SLUS601–MAY 2005
APPLICATION INFORMATION (continued)
The tracking startup feature of the TPS40100 is a separate control loop that controls the output voltage to a
reference applied to the TRKIN pin. This reference voltage is typically a ramp generated by an external R-C
circuit. Connecting the junction of R5, C5 and R6 (see Figure 10) of multiple converters together allows the
converters output voltages to track together during start up. A controlled power down is accomplished by pulling
down the common junction in a controlled manner and then removing power to the converters or turning them off
by grounding the UVLO pin.The relevant circuit fragment is shown in Figure 10.
Figure 10. Tracking Loop Control Schematic
First, select a value for R
4
. In order for this circuit to work properly, the output of the tracking amplifier must be
able to cause the FB pin to reach at least 690 mV with the output voltage at zero volts. This is so that the output
voltage can be forced to zero by the tracking amplifier. This places a maximum value on R
4
:
where
• V
HTRKOUT(min)
is the minimum output voltage of the tracking amplifier (see Electrical Characteristics table)
• V
DIODE
is the forward voltage of the device selected for D
1
• V
FB
is the value of the reference voltage (690 mV)
R
4
should not be chosen much lower than this value since that unnecessarily increases tracking loop gain,
making compensation more difficult and opening the door to potential non-linear control issues. D1 could be a
schottky if the impedance of the R
1
-R
BIAS
string is low enough that the leakage current is not a consequence. Be
aware that schottky diode leakage currents rise significantly at elevated temperature. If elevated temperature
operation and increased accuracy are important, use a standard or low leakage junction diode or the
base-emitter junction of a transistor for D
1
.
Once R
4
is selected, the gain of the closed loop power supply looking into “A” is known. That gain is the ratio of
R
1
and R
4
:
The tracking loop itself should have a crossover frequency much less that the crossover frequency of the voltage
control loop. Typically, the tracking loop crossover frequency is 1/10th or less of the voltage loop crossover
frequency to avoid loop interactions. Note that the presence of the diode in the circuit gives a non-linear control
mechanism for the tracking loop. The presence of this non-linearity makes designing a control loop more
challenging. The simplest approach is to simply limit the bandwidth of this loop to no more than necessary.
20