LTC2926IUFD View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2926IUFD' PDF : 28 Pages View PDF
LTC2926
APPLICATIO S I FOR ATIO
500mV/DIV
MASTER
SLAVE2
SLAVE1
500mV/DIV
5ms/DIV
5ms/DIV
2926 F13
Figure 13. Coincident Tracking Waveforms from Figure 14 Circuit
before the master ramp reaches its final value; otherwise,
the slave supply voltage will be held below its intended
level. Calculate the upper track resistor, RTB, from:
RTB
=
RFB
•
⎛
⎝⎜
SM
SS
⎞
⎠⎟
(4)
Choose a voltage difference between the master and slave
ramps, ΔV, if offset tracking is desired. If a time delay is
desired for supply sequencing, calculate an effective volt-
age difference based on the master ramp rate. If neither
voltage offset nor time delay is required, set ΔV = 0V.
ΔV = a voltage difference (offset tracking), or (5a)
ΔV = SM • tDLY (supply sequencing), or
(5b)
ΔV = 0V (coincident/ratiometric tracking)
(5c)
Use the following formula to determine the lower track
resistors, RTA, using the TRACK pin voltage, VTRACK, and
the FB pin internal reference voltage, VFB(REF), both from
the Electrical Characteristics:
RTA
=
VFB(REF)
+
VTRACK
VFB(REF) − VTRACK
+
∆V
(6)
RFB
RFA
RTB RTB
Note that large ratios of slave ramp rate to master ramp
rate, SS/SM, may result in negative values for RTA. In such
cases increase the offset or delay, or reduce the slave ramp
rate to realize positive values of RTA.
Coincident Tracking Example
A typical three-supply application is shown in Figure 14.
The master signal is 3.3V, the slave 1 supply is a 1.8V
module, and the slave 2 supply is a 2.5V module. Allow for
±10% tolerance of the slave supply voltages. Both slave
supplies track coincidently with the 3.3V master supply
that is controlled by an external MOSFET. The ramp rate of
the supplies is 100V/s. The slave supplies’ minimum load
resistances are 150Ω. The external configuration resistors
1.8V MODULE
3.3V VIN
3.3V IN OUT
SENSE
RX1
100Ω
Q0
IRF7413Z
Q1
IRF7413Z
10Ω
10Ω
3.3V
MASTER
1.8V
SLAVE1
2.5V MODULE
3.3V IN OUT
SENSE
RX2
100Ω
0.1µF
CMGATE
0.1µF
Q2
IRF7413Z
10Ω
2.5V
SLAVE2
RTB1
15.0k
RTB2
15.0k
RTA1
9.53k
RTA2
VIN
5.76k
10k
FAULT
ON/OFF
VCC
D1
MGATE RAMP SGATE1 SGATE2
D2
S1
RAMPBUF
S2
TRACK1
FB1
LTC2926
TRACK2
FB2
FAULT
ON
GND
STATUS/PGI
PGTMR
RSGATE
CPGTMR
1µF
2926 F14
RFB1
15.0k
RFA1
9.53k
RFB2
15.0k
VIN
10k
RFA2
5.76k
STATUS
NC
Figure 14. Coincident Tracking Example
2926fa
18
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