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Part Number LTC2925

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background image
LTC2925
1
2925f
APPLICATIO S
U
FEATURES
DESCRIPTIO
U
TYPICAL APPLICATIO S
U
The LTC
®
2925 provides a simple solution to power supply
tracking and sequencing requirements. By selecting a few
resistors, the supplies can be configured to ramp-up and
ramp-down together or with voltage offsets, time delays
or different ramp rates.
The LTC2925 controls the outputs of three independent
supplies without inserting any pass element losses. For
systems that require a fourth supply, or when a supply
does not allow direct access to its feedback resistors, one
supply can be controlled with a series FET. When the FET
is used, an internal remote sense switch compensates for
the voltage drop across the FET and current sense resistor,
and an electronic circuit breaker provides protection from
short-circuit conditions.
The LTC2925 also includes a power good timeout feature
that turns off the supplies if an external supply monitor
fails to indicate that the supplies have entered regulation
within an adjustable time-out period.
s
V
CORE
and V
I/O
Supply Tracking
s
Microprocessor, DSP and FPGA Supplies
s
Servers
s
Communication Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
s
Flexible Power Supply Tracking Up and Down
s
Power Supply Sequencing
s
Supply Stability Is Not Affected
s
Controls Three Supplies Without Series FETs
s
Controls an Optional Fourth Supply With a
Series FET
s
Electronic Circuit Breaker
s
Remote Sense Switch Compensates for Voltage
Drop Across a Series FET
s
Supply Shutdown Outputs
s
FAULT Output
s
Adjustable Power Good Timeout
s
Available in Narrow 24-Lead SSOP and Tiny 24-Lead
QFN Packages
Multiple Power Supply
Tracking Controller with
Power Good Timeout
0.015
Si4412ADY
0.1
µ
F
0.1
µ
F
0.82
µ
F
10k
V
CC
SENSEP SENSEN
3.3V
V
IN
3.3V
154k
3.3V V
IN
1.65k
88.7k
41.2k
2.5V
SLAVE2
MASTER
88.7k
41.2k
86.6k
100k
13k
100k
RAMPBUF
TRACK1
TRACK2
TRACK3
FB2
GATE
LTC2925
PGTMR
0.082
µ
F
SDTMR
0.47
µ
F
SCTMR
GND
2925 TA01
RAMP
100k
1.5V
SLAVE3
86.6k
DC/DC
IN
FB = 0.8V
OUT
DC/DC
IN
FB = 0.8V
RUN/SS
RUN/SS
OUT
FB3
SD3
SD2
3.3V
PGI
35.7k
10
1.8V
SLAVE1
16.5k
SUPPLY
MONITOR
DC/DC
IN
FB = 1.235V
RUN/SS
OUT
FB1
SD1
FAULT
ON
10k
V
IN
STATUS
REMOTE
RST
1V/DIV
3.3V
2.5V
1.8V
1.5V
10ms/DIV
2925 TA02a
1V/DIV
3.3V
2.5V
1.8V
1.5V
2925 TA02b
10ms/DIV
background image
LTC2925
2
2925f
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
CC
Input Supply Range
q
2.9
5.5
V
I
CC
Input Supply Current
I
FBx
= 0, I
TRACKx
= 0, I
RAMPBUF
= 0
q
1.5
3
mA
I
FBx
= ­1mA, I
TRACKx
= ­1mA,
q
10.5
15
mA
I
RAMPBUF
= ­3mA
V
CC(UVL)
Input Supply Undervoltage Lockout
V
CC
Rising
q
2.3
2.5
2.7
V
PACKAGE/ORDER I FOR ATIO
U
U
W
Supply Voltage (V
CC
) ................................. ­0.3V to 10V
Input Voltages
ON, PGI, SENSEP, SENSEN ..................... ­0.3V to 10V
TRACK1, TRACK2, TRACK3 .......... ­0.3V to V
CC
+ 0.3V
SCTMR, SDTMR, PGTMR ............. ­0.3V to V
CC
+ 0.3V
Output Voltages
FAULT, SD1, SD2, SD3,
FB1, FB2, FB3, STATUS ......................... ­0.3V to 10V
RAMPBUF, REMOTE ..................... ­0.3V to V
CC
+ 0.3V
RAMP .............................................. ­0.3V to V
CC
+ 1V
ORDER PART
NUMBER
UF PART
MARKING
T
JMAX
= 125
°
C,
JA
= 85
°
C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
2925
2925
LTC2925CUF
LTC2925IUF
ABSOLUTE AXI U
RATI GS
W
W
W
U
(Notes 1, 2)
ELECTRICAL CHARACTERISTICS
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
°
C. V
CC
= 5V unless otherwise noted.
GATE (Note 3) ........................................ ­0.3 to 11.5V
Average Current
TRACK1, TRACK2, TRACK3 .................................. 5mA
FB1, FB2, FB3 ....................................................... 5mA
Operating Temperature Range
LTC2925C ............................................... 0
°
C to 70
°
C
LTC2925I ............................................. ­40
°
C to 85
°
C
Storage Temperature Range .................. ­65
°
C to 150
°
C
Lead Temperature (Soldering, 10 sec)
MS Package ..................................................... 300
°
C
ORDER PART
NUMBER
GN PART
MARKING
LTC2925CGN
LTC2925IGN
LTC2925CGN
LTC2925IGN
24 23 22 21 20 19
7
8
9
TOP VIEW
UF PACKAGE
24-LEAD (4mm
×
4mm) PLASTIC QFN
EXPOSED PAD (PIN 25) INTERNALLY CONNECTED
TO GND (PCB CONNECTION OPTIONAL)
10 11 12
6
5
4
3
2
1
13
14
15
16
17
18
25
ON
SDTMR
SD1
SD2
SD3
RAMPBUF
SENSEN
SENSEP
V
CC
SCTMR
PGTMR
PGI
GND
FB3
TRACK3
FB2
TRACK2
TRACK1
STATUS
FAULT
GATE
RAMP
REMOTE
FB1
T
JMAX
= 125
°
C,
JA
= 37
°
C/W
1
2
3
4
5
6
7
8
9
10
11
12
TOP VIEW
GN PACKAGE
24-LEAD PLASTIC SSOP
24
23
22
21
20
19
18
17
16
15
14
13
V
CC
SENSEP
SENSEN
ON
SDTMR
SD1
SD2
SD3
RAMPBUF
GND
FB3
TRACK3
SCTMR
PGTMR
PGI
STATUS
FAULT
GATE
RAMP
REMOTE
FB1
TRACK1
TRACK2
FB2
background image
LTC2925
3
2925f
V
CC(UVL, HYST)
Input Supply Undervoltage Lockout Hysteresis
25
mV
V
GATE
External N-Channel Gate Drive (V
GATE
­ V
CC
)
I
GATE
= ­1
µ
A
q
5
5.5
6
V
I
GATE
GATE pin current
Gate On, V
GATE
= 0V, No Faults
q
­7
­10
­13
µ
A
Gate Off, V
GATE
= 5V, No Faults
q
7
10
13
µ
A
Gate Off, V
GATE
= 5V,
q
5
20
50
mA
Short-Circuit or Power Good Timeout
V
ON(TH)
ON Pin Threshold Voltage
V
ON
rising
q
1.214
1.232
1.250
V
V
ON(HYST)
ON Pin Hysteresis
q
30
75
150
mV
V
ON(FC)
ON Pin Fault Clear Threshold Voltage
q
0.3
0.4
0.5
V
I
ON(IN)
ON Pin Input Current
V
ON
= 1.2V, V
CC
= 5.5V
q
0
±
100
nA
V
RS-SENSE(TH)
Sense Resistor Over-Current Voltage Threshold 1V < V
SENSEP
< V
CC
q
40
50
60
mV
(V
SENSEP
­ V
SENSEN
)
0V < V
SENSEP
< 1V
q
30
50
70
mV
I
SENSEN
SENSEN Pin Input Current
0V < V
SENSEN
< V
CC
q
­1
5
10
µ
A
I
SENSEP
SENSEP Pin Input Current
0V < V
SENSEP
< V
CC
q
­1
5
10
µ
A
V
OS
Ramp Buffer Offset (V
RAMPBUF
­ V
RAMP
)
V
RAMPBUF
= V
CC
/2, I
RAMPBUF
= 0A
q
­30
0
30
mV
V
FAULT(OL)
FAULT Output Low Voltage
I
FAULT
= 3mA
q
0.2
0.4
V
V
SDx(OL)
SDx Output Low Voltage
I
SDx
= 1mA, V
CC
= 2.3
q
0.2
0.4
V
V
STATUS(OL)
STATUS Output Low Voltage
I
STATUS
= 3mA
q
0.2
0.4
V
I
RAMP
RAMP Pin Input Current
0V < RAMP < V
CC,
V
CC
= 5.5V
q
0
±
1
µ
A
V
RAMPBUF(OL)
RAMPBUF Low Voltage
I
RAMPBUF
= 3mA
q
90
150
mV
V
RAMPBUF(OH)
RAMPBUF High Voltage (V
CC
­ V
RAMPBUF
)
I
RAMPBUF
= ­3mA
q
100
200
mV
I
ERROR(%)
I
FBx
to I
TRACKx
Current Mismatch
I
TRACKx
= ­10
µ
A
q
0
±
5
%
I
ERROR(%)
= (I
FBx
­ I
TRACKx
)/I
TRACKx
I
TRACKx
= ­1mA
q
0
±
5
%
V
TRACKx
TRACK pin voltage
I
TRACKx
= ­10
µ
A
q
0.78
0.8
0.82
V
I
TRACKx
= ­1mA
q
0.78
0.8
0.82
V
I
FB(LEAK)
I
FB
Leakage Current
V
FB
= 1.5V, V
CC
= 5.5V
q
±
10
nA
V
FB(CLAMP)
V
FB
Clamp Voltage
1
µ
A < I
FB
< 1mA
q
1.6
2.1
2.5
V
R
REMOTE
REMOTE Feedback Switch Resistance
2V < V
REMOTE
< V
CC
q
15
30
I
SCTMR(UP)
Short-Circuit Timer Pullup Current
V
SCTMR
= 1V
q
­35
­50
­65
µ
A
I
SCTMR(DN)
Short-Circuit Timer Pulldown Current
V
SCTMR
= 1V
q
1
2
3
µ
A
V
SCTMR(TH)
Short-Circuit Timer Threshold Voltage
q
1.1
1.23
1.4
V
I
SDTMR(UP)
Shutdown Timer Pullup Current
V
SDTMR
= 1V
q
­7
­10
­13
µ
A
V
SDTMR(TH)
Shutdown Timer Threshold Voltage
q
1.1
1.23
1.4
V
I
PGI(UP)
Power Good Input Pullup Current
V
PGI
= 0V
q
­5
­10
­15
µ
A
V
PGI(TH)
Power Good Input Threshold Voltage
q
0.8
1.4
V
I
PGTMR(UP)
Power Good Timer Pullup Current
V
PGTMR
= 1V
q
­8
­10
­14
µ
A
V
PGTMR(TH)
Power Good Timer Threshold Voltage
q
1.1
1.23
1.4
V
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
ELECTRICAL CHARACTERISTICS
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
°
C. V
CC
= 5V unless otherwise noted.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: All currents into the device pins are positive; all currents out of
device pins are negative. All voltages are referenced to ground unless
otherwise specified.
Note 3: The GATE pin is internally limited to a minimum of 11.5V. Driving
this pin to voltages beyond the clamp may damage the part.
background image
LTC2925
4
2925f
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
V
GATE
vs V
CC
V
CC
(V)
2
8
V
GATE
(V)
9
10
12
11
3
4
2925 G03
5
6
Specifications are at T
A
= 25
°
C.
V
CC
(V)
2.9
I
CC
(mA)
1.3
1.4
1.5
5
2925 G01
1.2
1.1
1.0
3.5
4
4.5
5.5
I
TRACKx
= I
FBx
= 0mA
I
RAMPBUF
= 0mA
I
CC
vs V
CC
V
CC
(V)
0
0
V
SDx(OL)
(V)
0.2
0.4
0.6
0.8
1.0
1
2
3
4
2925 G02
5
I
SDx
= 10
µ
A
I
SDx
= 5mA
V
SDx(OL)
vs V
CC
PI FU CTIO S
U
U
U
GN/UF Packages
V
CC
(Pin 1/Pin 22): Positive Supply Input. The operating
supply input range is 2.9V to 5.5V. An undervoltage
lockout circuit resets the part when the supply is below
2.5V. V
CC
should be bypassed to GND with a 0.1
µ
F
capacitor.
SENSEP (Pin 2/Pin 23): Circuit Breaker Positive Sense
Input. SENSEP and SENSEN measure the voltage across
the sense resistor and trigger the circuit breaker function
when the current exceeds the level programmed by the
sense resistor for longer than a short circuit timer cycle
(see SCTMR). If unused, tie SENSEN and SENSEP to V
CC
.
SENSEN (Pin 3/Pin 24): Circuit Breaker Negative Sense
Input. SENSEN connects to the low side of the current
sense resistor. SENSEP and SENSEN monitor the current
through the external FET by measuring the voltage across
the sense resistor. The circuit breaker turns off the FET
when the sense voltage exceeds 50mV for longer than a
short circuit timer cycle (see SCTMR). If the short-circuit
timer times out, the GATE pin will be pulled low immedi-
ately to protect the FET. If unused, tie SENSEN and
SENSEP to V
CC
.
ON (Pin 4/Pin 1): On Control Input. The ON pin has a
threshold of 1.23V with 75mV of hysteresis. An active high
will cause 10
µ
A to flow from the GATE pin, ramping up the
supplies. An active low pulls 10
µ
A from the GATE pin,
ramping the supplies down. Pulling the ON pin below 0.4V
resets the electronic circuit breaker in the LTC2925. If a
resistive divider connected to V
CC
drives the ON pin, the
supplies will automatically start up when V
CC
is fully
powered.
SDTMR (Pin 5/Pin 2): Shutdown Timer. A capacitor from
SDTMR to GND sets the delay time between the ON pin
transitioning high (which releases the SDx pins) and the
supplies beginning to ramp-up. Float SDTMR when it is
unused.
SD1, SD2, SD3 (Pins 6, 7, 8/Pins 3, 4, 5): Outputs for
Slave Supply Shutdowns. The SDx pins are open-drain
outputs that hold the shutdown (RUN/SS) pins of the slave
supplies low until the ON pin is pulled above 1.23V. The
SDx pins will be pulled low again when RAMP <100mV and
ON <1.23V. If a slave supply is capable of operating with
an input supply that is lower than the LTC2925's minimum
operating voltage of 2.9V, the SDx pins can be used to hold
off the slave supplies. Each SDx pin is capable of sinking
greater than 1mA with supplies as low as 2.3V.
RAMPBUF (Pin 9/Pin 6): Ramp Buffer Output. Provides a
low impedance buffered version of the signal on the RAMP
pin. This buffered output drives the resistive dividers that
connect to the TRACKx pins. Limit the capacitance at the
RAMPBUF pin to less than 100pF.
background image
LTC2925
5
2925f
PI FU CTIO S
U
U
U
GN/UF Packages
GND (Pin 10/Pins 7, 25): Circuit Ground.
TRACK1, TRACK2, TRACK3 (Pins 15, 14, 12/Pins 12, 11,
9):
Tracking Control Input Pin. A resistive divider between
RAMPBUF, TRACKx and GND determines the tracking
profile of OUTx (see Applications Information). TRACKx
pulls up to 0.8V and the current supplied at TRACKx is
mirrored at FBx. The TRACKx pin is capable of supplying
at least 1mA when V
CC
= 2.9V. It may be capable of
supplying up to 10mA when the supply is at 5.5V, so care
should be taken not to short this pin for extended periods.
Limit the capacitance at the TRACKx pin to less than 25pF.
Float the TRACKx pins if unused.
FB1, FB2, FB3 (Pins 16, 13, 11/Pins 13, 10, 8): Feedback
Control Output. FBx connects to the feedback node of
slave supplies. Tracking is achieved by mirroring the
current from TRACKx into FBx. If the appropriate resistive
divider connects RAMPBUF and TRACKx, the FBx current
will force OUTx to track RAMP. The LTC2925 is capable of
controlling slave supplies with feedback voltages between
0V and 1.6V. To prevent damage to the slave supply, the
FBx pin will not force the slave's feedback node above
2.5V. In addition, it will not actively sink current from this
node even when the LTC2925 is unpowered. Float the FBx
pins if unused.
REMOTE (Pin 17/Pin 14): Remote Sense Switch. A 15
switch connects REMOTE to RAMP when the GATE is fully
enhanced (GATE > RAMP + 4.9V). Otherwise, it presents
a high-impedance. When the slave supplies track the
master supply, REMOTE can be used to compensate for
the voltage drop across the external sense resistor and N-
channel FET. A resistor between the output and the sense
nodes of the master supply provides feedback before the
external FET is fully enhanced. If an external FET is not
used, float REMOTE.
RAMP (Pin 18/Pin15): Ramp Buffer Input. When the
RAMP pin is connected to the source of the external N-
channel FET, the slave supplies track the FET's source as
it ramps up and down. Alternatively, when no external FET
is used, the RAMP pin can be tied directly to the GATE pin.
In this configuration, the supplies track the capacitor on
the GATE pin as it is charged and discharged by the 10
µ
A
current source controlled by the ON pin. When the GATE
is fully enhanced (GATE > RAMP + 4.9V) the open-drain
STATUS pin goes high-impedance and the remote sense
switch connects the RAMP pin to the REMOTE pin.
GATE (Pin 19/Pin 16): Gate Drive for External N-Channel
FET. When the ON pin is high, an internal 10
µ
A current
source charges the gate of the external N-channel MOSFET.
A capacitor connected from GATE to GND sets the ramp
rate. It is a good practice to add a 10
resistor between
this capacitor and the FET's gate to prevent high frequency
FET oscillations. An internal charge pump guarantees that
GATE will pull up to 5V above V
CC
ensuring that logic level
N-channel FETs are fully enhanced. When the ON pin is
pulled low, the GATE pin is pulled to GND with a 10
µ
A
current source. Under a short-circuit condition, the elec-
tronic circuit breaker in the LTC2925 pulls the GATE low
immediately with 20mA. Tie GATE to GND if unused.
FAULT (Pin 20/Pin 17): Circuit Breaker and Power Good
Timer Fault Output. FAULT is an open-drain output that
pulls low when the electronic circuit breaker is activated or
a power good timeout fault is detected. FAULT is reset by
pulling ON below 0.4V. To allow retry, tie FAULT to ON.
STATUS (Pin 21/Pin 18): Power Good Status Indicator.
The STATUS pin is an open-drain output that pulls low
until GATE has been fully charged at which time all
supplies will have reached their final operating voltage.
PGI (Pin 22/Pin 19): Power Good Timer Input. PGI con-
nects to the RST pin of the downstream supply monitor. If
PGI has not transitioned high within a power good timer
cycle (see PGTMR), the FAULT pin will be pulled low and
the supplies will be turned off by pulling the GATE pin low
with 20mA. PGI is pulled up with 10
µ
A. An internal
schottky diode allows PGI to be pulled safely above V
CC
.
Float PGI when it is unused.
PGTMR (Pin 23/Pin 20): Power Good Timer. A capacitor
from PGTMR to GND sets the Power Good Timer duration.
While ON > 1.23V, the PGTMR pin will pull up to V
CC
with
10
µ
A. Otherwise, it pulls to GND. If the voltage on the
PGTMR pin exceeds 1.23V and PGI is still low, FAULT will
be pulled low and the GATE will be pulled to ground with
20mA until the power good timer fault is cleared by pulling
ON below 0.4V. If FAULT is tied back to ON the system will
automatically retry after a FAULT. In this mode, verify that
the slave supplies' current limits provide sufficient protec-
tion under short-circuit conditions. Float PGTMR when it
is unused.