LTC2926CGN View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2926CGN' PDF : 28 Pages View PDF
LTC2926
APPLICATIO S I FOR ATIO
Q0
VIN
SUPPLY MODULE
VIN IN OUT
RX1
10Ω
SENSE
Q1
10Ω
MASTER
SLAVE1
SUPPLY MODULE
VIN IN OUT
RX2
SENSE
0.1µF CMGATE
Q2
10Ω
SLAVE2
RTB1
RTA1
RTB2
RTA2
VIN
10k
FAULT
ON/OFF
VCC
D1
MGATE RAMP SGATE1 SGATE2
D2
S1
RAMPBUF
S2
TRACK1
FB1
LTC2926
TRACK2
FB2
RFB1
RFA1
RFB2
VIN
RFA2
FAULT
ON
GND
PGTMR
STATUS/PGI
2926 F12
CPGTMR
10k
STATUS
Figure 12. Three-Supply Application
Three-Step Design Procedure
The following three-step design procedure allows one
to choose the FBn resistors, RFAn and RFBn, the TRACKn
resistors, RTAn and RTBn, and the master ramp capacitor,
CMGATE, that give any of the tracking or sequencing profiles
shown in Figures 1 to 4. A three-supply application circuit
is shown in Figure 12.
1. Set the ramp rate of the master signal.
Solve for the value of CMGATE, the capacitor on the MGATE
pin, based on the desired ramp rate (volts per second)
of the master ramp signal, SM, and the MGATE pull-up
current IMGATE, which is nominally 10µA.
CMGATE
=
IMGATE
SM
(1)
If the master ramp signal is a master supply, consider
the gate capacitance of the required external N-channel
MOSFET. If the gate capacitance is comparable to CMGATE,
reduce the external capacitor’s value to compensate for
the gate capacitance of the MOSFET.
If the master ramp signal is not a master supply, tie the
RAMP pin to the MGATE pin.
2. Choose the feedback resistors based on the slave
supply voltage and slave load.
It is important that the feedback resistors are significantly
larger than the load resistance, especially as the slave
voltage nears ground (see Load Requirements).
First determine the effective slave load resistance, RL (not
shown), at low slave voltage levels, and select the value
of the top feedback resistor, RFB, to satisfy:
RFB ≥ 100 • RL (recommended),
RFB ≥ 23 • RL (required)
(2)
Second, determine a value for the lower feedback resistor,
RFA, that will ensure that the LTC2926 fully enhances the
gate of the slave control MOSFET at the end of ramping.
Select RFA based on RFB, the resistor tolerance, TOLR, and
the maximum slave supply voltage, VSLAVE(max):
RFA
<
RFB
•
⎛
⎝⎜
1−
1+
TOLR
TOLR
⎞
⎠⎟
⎛
⎝⎜
VSLAVE(max)
0.784V
−
1 ⎞⎠⎟
(3)
Note: Choose the value of VSLAVE(max) to cover all slave
supply voltage tolerances by a good margin. Exceeding the
VSLAVE(max) voltage used for this calculation can result in
triggering a Power Good Fault unintentionally.
If the slave generator has an accessible resistive divider
and a ground-based voltage reference, it may be able to
be controlled without a series MOSFET. In that case, let
the generator’s design set RFA and RFB, substitute the
generator’s reference voltage for VFB(REF) in step 3, and
see the subsection Slave Control Without MOSFETs.
3. Solve for the tracking resistors that set the desired
ramp rate and voltage offset or time delay of the slave
supply.
Choose a ramp rate for the slave supply, SS. If the slave
supply tracks coincidently with the master supply or with
only a fixed offset or delay, then the slave ramp rate equals
the master ramp rate. Be sure that the slave ramp rate and
its offset or delay allows the slave voltage to finish ramping
2926fa
17
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