LTC2925CUF Linear Technology, LTC2925CUF Datasheet - Page 13

• Manufacturer Part Number: LTC2925CUF
• Description: IC POWER SUPPLY CONTROLLER 24QFN
• Manufacturer: Linear Technology

Specifications of LTC2925CUF

• Applications: Power Supply Controller
• Voltage - Supply: 2.9 V ~ 5.5 V
• Current - Supply: 1.5mA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 24-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Input: -

APPLICATIONS INFORMATION
3-Step Design Procedure
The following 3-step design procedure allows one to choose
the TRACK resistors, R
C
shown in Figures 1 to 4. A basic four supply application
circuit is shown in Figure 10.
1. Set the ramp rate of the master signal.
Solve for the value of C
pin, based on the desired ramp rate (V/s) of the master
supply, S
If the external FET has a gate capacitance comparable to
C
to compensate for the FET’s gate capacitance.
If no external FET is used, tie the GATE and RAMP pins
together, connect SENSEN and SENSEP to V
nect SCTMR to GND.
2. Solve for the pair of resistors that provide the desired
ramp rate of the slave supply, assuming no delay.
GATE
GATE
R
R
V
R
TB1
TA1
C
IN
TA2
GATE
V
V
, then the external capacitor’s value should be reduced
, that give any of the tracking or sequencing profi les
IN
IN
10k
10k
R
R
138k
R
100k
R
R
TB2
ONB
ONA
TB3
TA3
M
=
0.1μF
:
I
GATE
ON
REMOTE
STATUS
FAULT
S
V
RAMPBUF
TRACK1
TRACK2
TRACK3
Figure 10. Four Supply Application
GND
CC
M
SENSEP SENSEN
SCTMR
where I
R
SENSE
TAx
LTC2925
SDTMR
GATE
10Ω
and R
GATE
GATE
Q1
, the capacitor on the GATE
PGTMR
C
TBx
≈
GATE
10
, and the gate capacitor,
RAMP
SD1
SD3
SD2
FB1
FB2
FB3
PGI
μ
A
MASTER
R
R
R
2925 F10
RST
RUN/SS
FB
RUN/SS
FB
RUN/SS
FB
FA1
FA2
FA3
MONITOR
SUPPLY
DC/DC
DC/DC
DC/DC
R
R
CC
R
FB1
FB3
FB2
, and con-
V
V
V
OUT
OUT
OUT
IN
IN
IN
IN
IN
IN
SLAVE1
SLAVE2
SLAVE3
1 ( )
Choose a ramp rate for the slave supply, S
supply ramps up coincident with the master supply or
with a fi xed voltage offset, then the ramp rate equals the
master supply’s ramp rate. Be sure to use a fast enough
ramp rate for the slave supply so that it will fi nish ramping
before the master supply has reached its fi nal supply value.
If not, the slave supply will be held below the intended
regulation value by the master supply. Use the following
formulas to determine the resistor values for the desired
ramp rate, where R
the slave supply and V
of the slave supply:
where V
Note that large ratios of slave ramp rate to master ramp
rate, S
suffi ciently large delay is used in step 3, R
tive, otherwise S
3. Choose R
If no delay is required, such as in coincident and ratio-
metric tracking, then simply set R
is desired, as in offset tracking and supply sequencing,
calculate R
the desired delay.
the parallel combination of R
As noted in step 2, small delays and large ratios of slave
ramp rate to master ramp rate (usually only seen in se-
quencing) may result in solutions with negative values for
R
the ratio of slave ramp rate to master ramp rate must be
reduced.
TA
R
R
R
R
. In such cases, either the delay must be increased or
TB
TA
TA
TA
S
′ =
′′ =
=
/S
=
TRACK
R
R
M
R
TA
FB
V
TA
, may result in negative values for R
V
TA
FB
FB
TRACK
´´ to determine the value of R
•
′
≈ 0.8V.
t
to obtain the desired delay.
||
D
S
+
S
R
S
M
S
•
R
V
V
/S
TA
FB
S
TRACK
FB
FA
•
M
M
R
′′
and R
FB
must be reduced.
–
TB
is the feedback reference voltage
V
TRACK
R
FA
TB
TA
are the feedback resistors in
´ and R
TA
TA
= R
LTC2925
´´.
TA
TA
TA
S
. If the slave
´. If a delay
will be posi-
where t
TA
13
´. If a
2925fc
D
( ) 3
( ) 4
( ) 2
( ) 5
is