LTC2925C View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2925C' PDF : 24 Pages View PDF
LTC2925
APPLICATIONS INFORMATION
Tracking Cell
The LTC2925’s operation is based on the tracking cell
shown in Figure 5, which uses a proprietary wide-range
current mirror. The tracking cell shown in Figure 5 servos
the TRACK pin at 0.8V. The current supplied by the TRACK
pin is mirrored at the FB pin to establish a voltage at the
output of the slave supply. The slave output voltage varies
with the master signal, enabling the slave supply to be
controlled as a function of the master signal with terms
set by RTA and RTB. By selecting appropriate values of
RTA and RTB, it is possible to generate any of the profiles
in Figures 1 to 4.
VCC
MASTER
RTB
TRACK
RTA
+– 0.8V
FB
DC/DC
FB OUT
RFA RFB
2925 F05
Figure 5. Simplified Tracking Cell
SLAVE
Controlling the Ramp-Up and Ramp-Down Behavior
The operation of the LTC2925 is most easily understood by
referring to the simplified functional diagram in Figure 6.
When the ON pin is low, the GATE pin is pulled to ground
causing the master signal to remain low. Since the current
through RTB1 is at its maximum when the master signal is
low, the current from FB1 is also at its maximum. This cur-
rent drives the slave’s output to its minimum voltage.
When the ON pin rises above 1.23V, the master signal rises
and the slave supply tracks the master signal. The ramp
rate is set by an external capacitor driven by a 10μA current
source from an internal charge pump. If no external FET
is used, the ramp rate is set by tying the RAMP and GATE
pins together at one terminal of the external capacitor (see
the Ratiometric Tracking Example).
In a properly designed system, when the master signal
has reached its maximum voltage the current from the
TRACK1 pin is zero. In this case, there is no current from
the FB1 pin and the LTC2925 has no effect on the output
voltage accuracy, transient response or stability of the
slave supply.
When the ON pin falls below VON(TH) – ΔVON(HYST), typi-
cally 1.225V, the GATE pin pulls down with 10μA and the
master signal and the slave supplies will fall at the same
rate as they rose previously.
The ON pin can be controlled by a digital I/O pin or it
can be used to monitor an input supply. By connecting a
resistive divider from an input supply to the ON pin, the
supplies will ramp up only after the monitored supply has
reached a preset voltage.
VCC
SCTMR
50μA
2μA
SENSEP – SENSEN > 50mV
SENSEP
50mV
SENSEN
+
1.2V –
RONB
ON
+
RONA
1.2V
–
RAMPBUF
1s
10μA
10μA
VCC
GATE
RAMP
Q1
CGATE
MASTER
0.8V
RTB1
RTA1
TRACK1
FB1
DC/DC
SLAVE1
RFA1 RFB1
2925 F06
Figure 6. Simplified Functional Block Diagram
2925fc
9
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