LTC2921/LTC2922 Series
Power Supply Tracker
with Input Monitors
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FEATURES
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DESCRIPTIO
Tracks Multiple Supplies with MOSFET Switches
Monitors 5 Input Voltages Including VCC
Guaranteed Threshold Accuracy: ±1% at 0.5V
Automatic Remote Sense Switching
Adjustable Supply Ramp Rate
Overvoltage Monitor
Adjustable Electronic Circuit Breaker
Adjustable Power-Good Delay
Available for VCC Supply Voltages of 5V, 3.3V and 2.5V
Available in 16-Pin Narrow SSOP (LTC2921 Series)
and 20-Pin TSSOP (LTC2922 Series)
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APPLICATIO S
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Desktop Computers
Plug-In Cards
Telecom Infrastructure
Supply Sequencing
Instruments
The LTC®2921 and LTC2922 monitor up to five supplies
and force them to track on power-up in multiple supply
systems. Using external N-channel pass transistors, the
supplies can be ramped up at an adjustable rate. Automatic remote sense switching allows the DC/DC converters to compensate for series voltage drops in the wiring.
An incorrect level on one or more of the supplies triggers
disconnect of all supplies. Tight 1% accuracy and glitch
immunity on the low 0.5V monitoring level ensure no false
error disconnects.
The LTC2921 and LTC2922 each feature an adjustable
electronic circuit breaker to protect the VCC supply against
short circuits. Capacitance at the TIMER pin programs the
delays in the monitoring sequence.
The LTC2921 includes three remote sense switches in a
16-pin narrow SSOP package, while the LTC2922 includes
five remote sense switches in a 20-pin TSSOP package.
Both parts are available for VCC supply voltages of 5V,
3.3V, and 2.5V.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATIO
Three-Supply Tracker and Monitor (5V, 3.3V, 2.5V)
5V SUPPLY
VOUT
VFB
WSL1206
0.05Ω
100Ω
Si2316DS
DC/DC
CONVERTER
VFB
Si2316DS
100Ω
DC/DC
CONVERTER
3.3V
LOAD
2.5V SUPPLY
2V/DIV
10Ω
2.5V SUPPLY
VOUT
Load Voltage Ramp-Up and
Power-Good Activation
10Ω
3.3V SUPPLY
VOUT
5V
LOAD
VFB
Si2316DS
100Ω
DC/DC
CONVERTER
5V LOAD
2.5V
LOAD
3.3V LOAD
OUTPUTS
2V/DIV
2.5V LOAD
10Ω
243k
169k
VCC
49.9k
Si1012R
CBRST
100k
49.9k
V1
V2
V3
V4
0.47µF
LTC2921
PG
S1
S2
S3
PG
2V/DIV
100ms/DIV
5V SUPPLY AT 5V
3.3V SUPPLY AT 3.3V
2921/22 TA01b
RESET
D1
D2
D3
GND
CIRCUIT BREAKER
RESET CONTROL
4.7k
SENSE
GATE
TIMER
0.22µF
tGATE ~ 500ms
tTIMER ~ 130ms
2921/22 TA01
29212fa
1
LTC2921/LTC2922 Series
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ABSOLUTE
AXI U RATI GS (Note 1)
VCC Supply Voltage ...................................... –0.3V to 7V
V1, V2, V3, V4 Voltages ............................... –0.3V to 7V
SENSE Voltage ............................................ –0.3V to 7V
TIMER Voltage ............................. –0.3V to (VCC + 0.3V)
Charge Pumped Output Voltages
GATE, PG ............................................ –0.3V to 12.2V
Switch Voltages
S0, D0, S4, D4 (LTC2922 Series) ............ –0.3V to 7V
S1, D1, S2, D2, S3, D3 ............................ –0.3V to 7V
Switch Currents (DC, RMS)
S0, D0, S4, D4 (LTC2922 Series) ..................... 30mA
S1, D1, S2, D2, S3, D3 ..................................... 30mA
Operating Ambient Temperature Range
LTC2921C/LTC2922C .............................. 0°C to 70°C
LTC2921I/LTC2922I ............................–40°C to 85°C
Junction Temperature (Note 2) ............................. 125°C
Storage Temperature Range ..................–65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
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PACKAGE/ORDER I FOR ATIO
TOP VIEW
V1 1
16 TIMER
V2 2
15 VCC
V3 3
14 SENSE
V4 4
13 GATE
S3 5
12 PG
D3 6
11 GND
S2 7
10 D1
D2 8
9
S1
GN PACKAGE
16-LEAD NARROW PLASTIC SSOP
TJMAX = 125°C, θJA = 110°C/W
ORDER PART
NUMBER
LTC2921CGN
LTC2921CGN-3.3
LTC2921CGN-2.5
LTC2921IGN
LTC2921IGN-3.3
LTC2921IGN-2.5
GN PART MARKING
2921
292133
292125
2921I
921I33
921I25
ORDER PART
NUMBER
TOP VIEW
SO 1
TIMER 2
20 D0
19 VCC
V1 3
18 SENSE
V2 4
17 GATE
V3 5
16 PG
V4 6
15 GND
S4 7
14 D1
D4 8
13 S1
S3 9
12 D2
D3 10
11 S2
LTC2922CF
LTC2922CF-3.3
LTC2922CF-2.5
LTC2922IF
LTC2922IF-3.3
LTC2922IF-2.5
F PACKAGE
20-LEAD PLASTIC TSSOP
TJMAX = 125°C, θJA = 90°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V for LTC2921/LTC2922, VCC = 3.3V for
LTC2921-3.3/LTC2922-3.3, and VCC = 2.5V for LTC2921-2.5/LTC2922-2.5, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Supply Voltage
Typical Operating Range
LTC2921/LTC2922
LTC2921-3.3/LTC2922-3.3
LTC2921-2.5/LTC2922-2.5
4.50
2.97
2.37
5.00
3.30
2.50
5.50
3.63
2.63
V
V
V
Supply Pin
VCC
ICC
Supply Current
VCC(MON)
Supply Monitor Threshold Voltage
LTC2921/LTC2922
LTC2921-3.3/LTC2922-3.3
LTC2921-2.5/LTC2922-2.5
●
●
●
4.285
2.828
2.265
4.350
2.871
2.300
2
4.415
2.914
2.335
mA
V
V
V
VCC(OV)
Supply Overvoltage Threshold
LTC2921/LTC2922
LTC2921-3.3/LTC2922-3.3
LTC2921-2.5/LTC2922-2.5
●
●
●
5.82
3.84
3.08
6.13
4.04
3.24
6.43
4.24
3.40
V
V
V
29212fa
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LTC2921/LTC2922 Series
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V for LTC2921/LTC2922, VCC = 3.3V for
LTC2921-3.3/LTC2922-3.3, and VCC = 2.5V for LTC2921-2.5/LTC2922-2.5, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
VCC(UVLO)
Supply Undervoltage Lockout
VCC Rising
VCC(UVH)
Supply Undervoltage Hysteresis
VCC Falling
●
MIN
TYP
MAX
2.08
2.20
2.30
UNITS
V
120
mV
Electronic Circuit Breaker
∆VSENSE
Circuit Breaker Trip Voltage
ISENSE
SENSE Pin Input Current
tV1(DLY)
Circuit Breaker Trip Delay Time
tV1(RST)
VV1(RST)
Circuit Breaker Reset Pulse Width
∆VSENSE = VCC - VSENSE
●
VCC - VSENSE = 150mV
LTC2921/LTC2922
LTC2921-3.3/LTC2922-3.3
LTC2921-2.5/LTC2922-2.5
Guaranteed Not to Reset
Guaranteed to Reset
Circuit Breaker Reset Threshold Voltage
45
0.5
0.5
0.5
50
55
mV
150
500
nA
1.5
1.5
1.5
3.0
3.0
6.0
µs
µs
µs
50
µs
µs
●
●
150
●
0.490
0.500
0.510
V
●
0.495
0.492
0.500
0.500
0.505
0.508
V
V
●
0.665
0.700
0.735
V
±0.1
µA
Monitor Inputs
VMON
V1-V4 Monitor Threshold Voltages
VOV
V1-V4 Overvoltage Thresholds
IMON
V1-V4 Input Currents
TIMER Pin
VTIMER(TH)
TIMER Ramp Threshold Voltage
●
1.15
1.20
1.25
V
ITIMER(PU)
TIMER Pull-Up Current
VTIMER = 1V
ITIMER(PD)
TIMER Pull-Down Current
VCC = 2.35V, VTIMER = 0.4V
●
–1.3
–2.0
–2.5
µA
VTIMER(CLR)
TIMER Cleared Threshold Voltage
VTIMER Falling
VGATE
GATE Drive Output Voltage
LTC2921/LTC2922
LTC2921-3.3/LTC2922-3.3
LTC2921-2.5/LTC2922-2.5
●
●
●
IGATE(PU)
GATE Pull-Up Current
VGATE = VCC
●
IGATE(PD)
GATE Pull-Down Current
VCC = 2.35V, VGATE = 2.35V
µA
100
150
250
mV
10.0
8.4
6.1
11.1
9.1
6.8
12.2
9.8
7.5
V
V
V
–6.5
–10.0
–12.5
µA
GATE Pin
10
mA
Remote Sense Switches
RDS(FB)
Feedback Switch Resistances (Note 3)
VD = VCC
●
●
2
10
–4.0
–5.0
Ω
PG Pin
IPG(PU)
PG Pull-Up Current
VPG = VCC
IPG(PD)
PG Pull-Down Current
VCC = 2.35V, VPG = 2.35V
VPG(OL)
PG Output Low Voltage
VCC = 2.35V, IPG = 5mA
●
VPG
PG Output Voltage (Note 4)
LTC2921/LTC2922
LTC2921-3.3/LTC2922-3.3
LTC2921-2.5/LTC2922-2.5
●
●
●
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: TJ is calculated from the ambient temperature TA and power
dissipation PD as follows:
LTC2921 Series: TJ = TA + (PD • 110°C/W)
LTC2922 Series: TJ = TA + (PD • 90°C/W)
–2.6
µA
10
10.0
8.4
6.1
mA
11.1
9.1
6.8
0.4
V
12.2
9.8
7.5
V
V
V
Note 3: This specification applies to all switches, and is measured with
VS < VD.
Note 4: The PG pin will rise to approximately the same voltage as the
GATE pin when not pulled up or pulled down by external resistance.
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LTC2921/LTC2922 Series
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TYPICAL PERFOR A CE CHARACTERISTICS
Specifications are at TA = 25°C unless otherwise noted.
Supply Current vs Supply Voltage
3.00
Monitor Trip Delay vs
Monitor Input Overdrive
Supply Current vs Temperature
2.6
PG SIGNAL ASSERTED
100
PG SIGNAL ASSERTED
2.4
LTC2921
LTC2922
LTC2921-3.3
LTC2922-3.3
2.00
LTC2921
LTC2922
ICC (mA)
ICC (mA)
2.50
2.25
LTC2921-2.5
LTC2922-2.5
MONITOR TRIP DELAY (µs)
2.75
LTC2921-3.3
LTC2922-3.3
2.2
LTC2921-2.5
LTC2922-2.5
2.0
1.75
1.50
1.8
–50 –30
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
VCC (V)
80
60
LTC2921-2.5
LTC2922-2.5
40
LTC2921/LTC2922
LTC2921-3.3/LTC2922-3.3
20
0
30
50
–10 10
TEMPERATURE (°C)
70
90
80
40
100 120
60
20
MONITOR INPUT OVERDRIVE (mV)
0
2921/2 G03
2921/2 G02
2921/2 G01
Monitor Input Threshold vs
Temperature
SENSE Input Current
vs Temperature
Circuit Breaker Trip Voltage vs
Temperature
0.505
140
250
55
0.500
ISENSE (nA)
200
BREAKER TRIP (mV)
MONITOR INPUT THRESHOLD (V)
VSENSE = VCC
50
150
100
50
0.495
–50
–30
30
–10 10
50
TEMPERATURE (°C)
70
45
–50
90
–30
30
10
–10
50
TEMPERATURE (°C)
2921/2 G04
0
–50
90
30
–10 10
50
TEMPERATURE (°C)
2.5
1.21
2.4
90
TIMER Pull-Down Current
vs Supply Voltage
170
VTIMER = 1V
PULL-DOWN CURRENT (µA)
2.3
2.2
CURRENT (µA)
70
2921/2 G06
TIMER Pull-Up Current
vs Temperature
1.20
–30
2921/2 G05
TIMER Trip Voltage
vs Temperature
TIMER TRIP VOLTAGE (V)
70
2.1
2.0
1.9
1.8
1.7
VTIMER = 0.4V
165
160
155
150
145
1.6
1.19
–50
–30
30
–10 10
50
TEMPERATURE (°C)
70
90
1.5
–50
–30
–10
10
30
50
70
90
TEMPERATURE (°C)
2921/2 G07
2921/2 G08
140
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
VCC (V)
2921/2 G09
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LTC2921/LTC2922 Series
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TYPICAL PERFOR A CE CHARACTERISTICS
Specifications are at TA = 25°C unless otherwise noted.
Gate Voltage vs Supply Voltage
LTC2921-3.3
LTC2922-3.3
LTC2921-2.5
LTC2922-2.5
10
LTC2921-3.3
LTC2922-3.3
9
8
GATE LOAD = 1000pF || 10MΩ
PG LOAD = 2kΩ TO VCC
VCC BYPASS CAP = 1µF
LTC2921-2.5
LTC2922-2.5
7
6
6
–50 –30
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
VCC (V)
30
50
–10 10
TEMPERATURE (°C)
2921/2 G10
9
7
70
90
2921/2 G13
0
1
2
3 4 5 6 7 8
LOAD CURRENT (µA)
9
10
PG Pull-Up Current vs
Temperature
5.5
VPG = VCC
5.0
LTC2921-3.3
LTC2922-3.3
8
9.0
GATE LOAD = 1000pF || 10MΩ
PG LOAD = 2kΩ TO VCC
VCC BYPASS CAP = 1µF
2921/2 G12
CURRENT (µA)
PG (V)
CURRENT (µA)
10.5
30
50
–10 10
TEMPERATURE (°C)
0
90
GATE LOAD = 1000pF || 10MΩ
PG LOAD = 1000pF || 10MΩ
11 VCC BYPASS CAP = 1µF
LTC2921
LTC2922
10
11.0
8.5
–50 –30
4
2
12
VGATE = VCC
9.5
LTC2921-2.5
LTC2922-2.5
6
PG Voltage vs Supply Voltage
10.0
LTC2921-3.3
LTC2922-3.3
8
2921/2 G11
GATE Pull-Up Current
vs Temperature
11.5
70
LTC2921
LTC2922
10
LTC2921
LTC2922
GATE VOLTAGE (V)
9
7
12
11
GATE VOLTAGE (V)
GATE VOLTAGE (V)
12
GATE LOAD = 1000pF || 10MΩ
PG LOAD = 2kΩ TO VCC
11 VCC BYPASS CAP = 1µF
LTC2921
LTC2922
10
8
Gate Voltage vs Load Current
Gate Voltage vs Temperature
12
LTC2921-2.5
LTC2922-2.5
4.5
4.0
3.5
3.0
6
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
VCC (V)
2921/2 G14
2.5
–50 –30
30
50
–10 10
TEMPERATURE (°C)
70
90
2921/2 G15
29212fa
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LTC2921/LTC2922 Series
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PI FU CTIO S
(LTC2921/LTC2922 or [LTC2922 Only])
S0, D0 (Pins 1, 20 [LTC2922]): Remote Switch 0. These
pins are the terminals of an internal N-channel FET switch
that is enabled after the GATE pin is fully ramped up. This
switch can be used to connect a remote sense line to
compensate for IR drop across the external FETs. The gate
of the internal switch ramps up at a nominal rate of 8V/ms.
The pins are interchangeable, either switch pin can be tied
to the load side. Tie both pins to ground if unused.
S4, D4 (Pins 7, 8 [LTC2922]): Remote Sense Switch 4. Tie
to GND if unused.
S3, D3 (Pins 5, 6/Pins 9, 10): Remote Sense Switch 3. Tie
to GND if unused.
S2, D2 (Pins 7, 8/Pins 11, 12): Remote Sense Switch 2.
Tie to GND if unused.
S1, D1 (Pins 9, 10/Pins 13, 14): Remote Sense Switch 1.
Tie to GND if unused.
TIMER (Pin 16/Pin 2): Timing Delay Input. Connect a
capacitor between this pin and ground to set a 600ms/µF
delay at two points in the monitoring sequence. This sets
the delay after all monitors are good, before the start of
GATE ramping, and the delay after the remote sense
switches are on, before PG is activated. TIMER must fall
below 150mV before a timing delay can start. The TIMER
pin is pulled to ground at other points in the sequence.
V1-V4 (Pins 1-4/Pins 3-6): Supply Monitor Inputs. All four
inputs must lie above the monitor threshold level (0.5V)
and below the monitor overvoltage level (0.7V) for a turnon sequence to commence or continue. When any monitor
input falls outside those levels, the GATE and PG pins are
pulled low, disconnecting all the loads. Glitch filtering on
the 0.5V monitor threshold prevents low-energy voltage
spikes from affecting the comparators’ results. V1 also
serves as an active-low reset pin for the circuit breaker. Tie
unused monitor inputs to used monitor inputs.
GND (Pin 11/Pin 15): Circuit Ground.
PG (Pin 12/Pin 16): Power Good Output. A 4µA current
source from the internal charge pump rail (VPUMP) pulls
PG up after the turn-on sequence is complete. The output
is pulled to ground before turn-on is complete, when any
monitor is out of compliance, when the circuit breaker
trips, and when VCC is undervoltage. An external resistor
can be added to pull up to a lower voltage and to improve
pull up speed. This pin can also be configured as a gate
drive for external N-channel FETs in sequencing applications. In applications not requiring the PG output, leave the
pin unconnected.
GATE (Pin 13/Pin 17): Gate Drive for External N-Channel
FETs. A 10µA current source from the internal charge
pump rail (VPUMP) ramps the gates of the external Nchannel MOSFETs forcing all supplies to track on. The
resistor and capacitor network from this pin to ground
sets the supplies’ ramp rate and enhances control loop
stability.
SENSE (Pin 14/Pin 18): Circuit Breaker Sense Input. An
external resistor between VCC and SENSE sets the electronic circuit breaker trip current. The breaker trips when
the voltage across the resistor exceeds 50mV for 1µs. To
disable the circuit breaker tie SENSE to VCC. To reset the
circuit breaker after the current falls below the trip point,
pull the V1 pin below 0.5V for >150µs or go into
undervoltage lockout for >10µs.
VCC (Pin 15/Pin 19): Supply Voltage. The voltage at VCC is
monitored through an internal resistive divider in a manner similar to the V1-V4 inputs. An undervoltage lockout
circuit disables the part until the voltage at VCC is greater
than 2.2V. The VCC pin must be connected to the highest
supply voltage. Bypass the VCC pin to ground with a 10µF
capacitor.
29212fa
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LTC2921/LTC2922 Series
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FUNCTIONAL DIAGRA
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VPUMP
OVERCURRENT
–
LATCH
+
SENSE
50mV
+–
V
VCC
4µA
PG
ENABLE
UNDERVOLTAGE
–
VPUMP
OVERVOLTAGE
REMOTE
SENSE
SWITCH
GATE
+
+
REMOTE
SENSE
SWITCH
ENABLE
MONITOR
–
–
OVERVOLTAGE
+
+
V1
VPUMP
MONITOR
CONTROL
LOGIC
–
–
OVERVOLTAGE
+
10µA
GATE
GATE
ENABLE
+
V2
PG
GATE
ON
+
MONITOR
VCC
–
–
OVERVOLTAGE
2µA
VSWON
+
+
V3
TIMER
ENABLE
MONITOR
+
–
–
V4
TIMER
DONE
VCC
OVERVOLTAGE
+
+
TIMER
CIRCUIT
BREAKER
RESET PULSE
TIMER ENABLE
MONITOR
–
+
PULSE
TIMER
DONE
0.7V
0.5V
1.2V
+
–
VPUMP ≈ 11.1V
AT VCC =5V
APPROXIMATELY 1V
VSWON
1.2V
0.7V
0.5V
SWITCHES ON
+
VSWON
REFERENCE
GENERATOR
AND
CHARGE
PUMP
(LTC2922 ONLY)
D0
GND
S0
(LTC2922 ONLY)
D1
D2
D3
(LTC2922 ONLY)
D4
2921/22 F01
S1
S2
S3
S4
(LTC2922 ONLY)
Figure 1. LTC2921 and LTC2922 Functional Diagram
29212fa
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LTC2921/LTC2922 Series
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OPERATIO
General Operation
The LTC2921 and LTC2922 track multiple supplies, monitor multiple inputs, and provide integrated switches for
remote sensing. Once all input voltages lie between monitoring and overvoltage threshold levels, in-line FETs are
turned on to simultaneously ramp power to the loads. The
automatic remote sense switches are then activated, and
the power good signal is asserted. After initial power-on
the LTC2921 and LTC2922 continue monitoring the inputs. Several types of events will trigger interruption, any
of which will disconnect all supplies, deactivate the power
good signal, and open the remote sense switches.
Monitoring Sequence
A normal power-on sequence comprises the following
steps:
Step 0) Wait for VCC to exceed the undervoltage lockout
threshold. Continue checking VCC.
Step 1) Confirm that the circuit breaker has not tripped and
wait for all monitored supplies, including VCC, to be
between their programmed monitor and overvoltage
thresholds. Continue checking these conditions.
Step 2) Check that the TIMER pin voltage starts below
150mV. Create a delay by ramping up the TIMER pin until
it trips an internal comparator.
Step 3) Ramp the GATE pin to turn on the external
N-channel FETs, simultaneously ramping the supplies into
their loads. Await confirmation of full GATE enhancement,
i.e., GATE voltage within ~1V of VPUMP. Continue checking
this condition.
Step 4) Activate the remote sense switches. Await confirmation of full Feedback Switch Gate enhancement.
Step 5) Wait again for another TIMER cycle delay.
Step 6) Release the pull-down on the PG output. Continue
checking VCC, the circuit breaker, the input voltages, and
the GATE voltage.
Interrupting Events
Three events can interrupt the sequence and trigger immediate disconnect of all supplies, pull-down of the PG
signal, and deactivation of the remote sense switches. The
three interrupting events are a lockout, a fault, and an
error.
A lockout occurs when VCC falls below the undervoltage
threshold (including hysteresis). Escape from lockout
requires sufficient VCC voltage. Leaving lockout, the sequence begins at Step 1. A lockout condition supersedes
faults and errors.
A fault occurs when the circuit breaker trips. Escape from
a fault requires pulsing the V1 pin below the reset threshold of 0.5V(nom) for more than 150µs after the current
falls below the trip point. When V1 returns high, the
sequence begins from Step 1. An undervoltage lockout of
>10µs also clears the circuit breaker fault latch. A fault
condition supersedes errors.
An error occurs when one or more of the monitor inputs
(V1-V4 pins) or VCC falls below its monitor threshold, or
rises above its overvoltage threshold. A loss of voltage on
the GATE pin, once it has fully ramped up, also causes an
error. An error sends the sequence to Step 1.
Feedback Switches for Remote Sensing
The integrated N-channel switches of the LTC2921/
LTC2922 automatically compensate for the voltage drops
caused by the RDS(ON) of the external load-control MOSFET switches. This is accomplished by modifying the
normal feedback path of each power supply that is controlled by the LTC2921/LTC2922. When the load-control
switches are off, the remote sense switches are also off,
and the power supply uses its normal feedback path to
sense its output voltage. After the load-control switches
are turned on, the remote sense switches are turned on to
create dominating feedback paths. The feedback loops
include the load-control switches, thus compensating for
their voltage drops.
In order to eliminate glitching on the output of the power
supply, the remote sense switches are turned on at a
controlled rate of about 8V/ms. The gates of these integrated N-channel devices are pulled up above VCC to
VPUMP so as to provide a low-resistance path for a wide
range of voltages.
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Electronic Circuit Breaker
Placing a resistor between the VCC and SENSE pins allows
the part to detect shorts and excessive currents on the VCC
supply. The electronic circuit breaker trips when the
voltage across the resistor is >50mV for more than 1µs. A
trip causes a fault condition which interrupts the monitor
sequence, and which requires reset of the circuit breaker
latch (see Interrupting Events section). Breaker reset is
achieved by pulling V1 below the reset threshold for
>150µs after the current falls below the trip point, or by
returning from undervoltage lockout on VCC.
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The timing of a typical start-up sequence for the LTC2921/
LTC2922 is shown in Figure 2. VCC exceeds the undervoltage
lockout level at time 0. All monitor inputs settle between
the 0.5V monitor threshold and the 0.7V overvoltage
threshold by time 1, then a TIMER cycle starts. The TIMER
pin reaches 1.2V at time 2, and GATE ramping begins.
When the GATE ramp completes at time 3, the automatic
remote sense switches close. Another TIMER delay
begins at time 4 and finishes at time 5, at which time PG
is activated.
0
VCC
1
2
3
4
UNDERVOLTAGE
LOCKOUT LEVEL
5
The timing of a monitor failure and subsequent regular
turn-on is shown in Figure 3. Prior to time 1, a successful
turn-on sequence had completed. At time 1, monitor V2
falls below the 0.5V reference, triggering an error. The
GATE pin, PG pin, and the remote sense switches fall at
rates determined by the pull-down currents and loading
conditions of each (times 2, 3, 4). At time 5, monitor V2
recovers, and a normal turn-on sequence begins.
VCC
SENSE
1
23 4
5
UNDERVOLTAGE
LOCKOUT LEVEL
VCC
VCC
SENSE
VCC-50mV
VCC-50mV
V1
V1
0.7V
0.5V
V2
V2
V3
V4
0.7V
0.5V
0.7V
0.5V
0.7V
0.5V
0.7V
0.5V
V3
0.7V
0.5V
V4
0.7V
0.5V
0.7V
0.5V
0.7V
0.5V
1.2V
1.2V
TIMER
1.2V
1.2V
TIMER
GATE
GATE
VCC
REMOTE
SENSE
SWITCH
GATE
VCC
REMOTE
SENSE
SWITCH
GATE
PG
PG
Figure 2. Typical Start-Up Sequence
Figure 3. Monitor Failure and Start-Up Sequence Timing
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The timing of a circuit breaker trip and reset, and a
subsequent regular turn-on are shown in Figure 4. Prior to
time 1, a successful turn-on sequence had completed. At
time 1, excessive current pulls SENSE more than 50mV
below VCC. The GATE pin, PG pin, and the remote sense
switches fall at rates determined by the pull-down currents
VCC
SENSE
1
2 3 4
5
and loading conditions of each (times 2, 3, 4). Note that the
excessive current condition ceases at time 4. A circuit
breaker reset pulse is initiated at time 5. The latch resets
at time 6 since the V1 pulse is wide enough. A normal turnon begins when V1 rises above the monitor threshold
(time 7 onward).
6 7
UNDERVOLTAGE
LOCKOUT LEVEL
VCC
VCC
VCC-50mV
VCC-50mV
INTERNAL
CIRCUIT
BREAKER
LATCH
V1
0.7V
0.5V
V2
0.7V
0.5V
V3
0.7V
0.5V
V4
0.7V
0.5V
1.2V
1.2V
TIMER
VCC
GATE
REMOTE
SENSE
SWITCH
GATE
PG
Figure 4. Circuit Breaker Trip, Reset and Start-Up Sequence Timing
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Multiple supply systems have become common to accommodate circuits on the same board with different voltage
requirements. Desktop PC motherboards, instrumentation circuits and plug-in boards of all kinds often require
tracking and control of several supply voltages.
The LTC2921 and LTC2922 ramp and monitor up to five
supply voltages in such systems. External resistive voltage dividers independently program four monitor levels,
while an internal divider sets the VCC pin supply monitor
level. Time delays in the monitoring sequence are set by an
external capacitor at the TIMER pin.
The GATE pin provides a high side drive voltage appropriate to logic-level and sublogic-level N-channel power
MOSFETs. The external RC network on GATE programs
the supply ramp rate and eliminates possible high frequency oscillations in the power path. Featured in the
LTC2921/LTC2922 series are sub-10Ω internal remote
sense switches to compensate for voltage drops between
the supplies and the loads.
At the end of a successful power-on sequence, the LTC2921/
LTC2922 asserts the PG output. A typical application uses
an external pull-up resistor between PG and the load side
of a supply. In applications where supply power-on sequencing is required, the PG pin can function as a second,
separate high side driver.
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Setting the Supply Monitor Levels
RB1(MAX) =
The LTC2921 and LTC2922 series both feature low 0.5V
monitoring thresholds with tight 1% accuracy. To set a
supply monitoring level tightly, design a precision ratio
resistive divider to relate the lowest valid supply voltage to
the maximum specified monitor threshold voltage. Use
resistors with 1% tolerance or better to limit the error due
to mismatch. The basic resistive divider connection for
supply monitoring is shown in Figure 5.
1 – RTOL VSRC1(MIN) – 0.505V
R A1 •
•
1 + RTOL 0.505V + 0.1µA • RA1
+
VSRC1
VOUT
VQ1
–
VL1
RB1
RG1
10Ω
IMON
VFB
VV1
RZ1
GND
IA1
±0.1µA
RA1
V1
LTC2922
DC/DC
CONVERTER
CGATE
2921/22 F05
Figure 5. Basic Monitor Connection
First, divide the nominal monitor threshold voltage by an
acceptable bias current (IA1), and choose a nearby standard value for resistor RA1 (see Equation 1).
Next, calculate the bounds on the value of RB1 that
guarantee that the divided minimum supply voltage exceeds the maximum specified monitor threshold voltage,
and that the minimum specified overvoltage threshold
exceeds the divided maximum supply voltage. Use Equations 2 and 3 to calculate RB1(MAX) and RB1(MIN) from RA1,
the resistor tolerance (RTOL), the supply voltage, the
monitor threshold and overvoltage specifications, and the
monitor pin leakage current specification.
When the integrated remote sensing switch is closed, the
DC/DC converter will compensate for the IR drop from
drain to source of the external N-channel FET (VQ1(ON)) by
increasing the supply voltage by the same amount. Calculate with VQ1(ON)(MAX) = 0V if the remote sense switch is
not used.
R A1 =
0.500V
IA1
VSRC1(MAX) + VQ1(ON)(MAX) – 0.665V
0.665V – 0.1µA • RA1
RB1(MIN) ≤ RB1 ≤ RB1(MAX)
LOAD
GATE
GND
1 + RTOL
RB1(MIN) = RA1 •
•
1 – RTOL
(3)
Choose a standard resistor value for RB1 that satisfies the
inequality of Equation 4.
Q1
RY1
(2)
(1)
(4)
When several standard values meet the requirement,
choose the value closest to RB1(MAX) to set the tightest
monitor threshold. This also allows more headroom for
larger VQ1(ON)(MAX). Alternatively, choose the standard
value closest to RB1(MIN) to set the tightest overvoltage
threshold.
All four monitor input voltages must be between the
monitor threshold and the overvoltage threshold for the
turn-on sequence to begin. Connect unneeded monitor
input pins to any of the utilized monitor input pins.
Selecting the External N-Channel MOSFETs
The GATE pin drives the gate of external N-channel
MOSFETs above VCC to connect the supplies to the loads.
The GATE drive voltage provided by the LTC2921/LTC2922
series is best suited to logic-level and sublogic-level
power MOSFETs. To achieve the lowest switch resistance,
the VCC pin must be connected to the highest supply
voltage.
Consider the application requirements for current, turnoff
speed, on-resistance, gate-source voltage specification,
etc. Refer to the Electrical Specifications and Typical
Performance Curves to determine the GATE voltages for
given VCC voltages over the required range of conditions.
Calculate the minimum gate drive voltage for each monitored supply for use in selecting the FETs. Check the
maximum GATE voltage against the FETs’ gate-source
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voltage specifications. On-resistance is a critical parameter when choosing power MOSFETs. The integrated
remote sense switches compensate for IR drops, but
minimizing VQ(MAX) leaves more margin for designing the
resistive voltage divider for the monitors.
VSRC2
CD2
0.1µF
(OPT)
VSRC1
CD1
0.1µF
(OPT)
Setting the GATE Ramp Rate
Application of power to the loads is controlled by setting
the voltage ramping rate with an external capacitor on the
GATE pin. During Step 3 of the monitoring sequence, a
10µA pull-up ramps the GATE pin capacitance up to
VPUMP, the internal charge pump voltage. Use Equation 5
to calculate the nominal GATE pin capacitance necessary
to achieve a given ramp rate, ∆V/∆t:
C GATE =
10µA
∆V / ∆t
10µA • tRAMP
VGATE
CD0
0.1µF
(OPT)
VL2
RG2
10Ω
Q1
VL1
RG1
10Ω
Q0
VL0
RG0
10Ω
GATE
LTC2922
CGATE
GND
2921/22 F06
Figure 6. Ramping and Damping Components on GATE Pin
(5)
Alternatively, to calculate the GATE capacitor to achieve a
desired nominal ramp time, use Equation 6. The GATE
drive voltage (VGATE) varies with VCC voltage. Consult the
Electrical Characteristics table and Typical Performance
curves to choose an appropriate value to insert for VGATE.
C GATE =
VSRC0
Q2
(6)
When the GATE pin drives several FETs in parallel, the load
voltages ramp together at the same rate until the lowest
supply reaches its full value. The other supplies continue
to track until the next lowest supply reaches its full value,
and so on.
The GATE pin must not be forced above the level it reaches
when fully ramped. An internal clamp limits the GATE
voltage to ≤12.2V relative to ground.
Damp possible ramp-on oscillations by including a 10Ω
resistor in series with each external N-channel gate, and as
necessary, a 0.1µF capacitor on each external N-channel
drain, as shown in Figure 6.
Setting the Sequence Delay Timer
The turn-on sequence includes two programmable delays
set by the capacitance on the TIMER pin. More precisely,
a single delay value is used at two points in the sequence.
In both cases, the delay provides a measure of confidence
that conditions are stable enough for the sequence to
advance.
The first TIMER delay begins once all monitor voltages
comply with their thresholds, the electronic circuit breaker
has not tripped, and VCC is not undervoltage. The TIMER
pin sources 2µA into an external capacitor, which ramps
its voltage. A comparator trips when the TIMER pin voltage
reaches the internal 1.2V reference, then the GATE ramp
begins, and TIMER is pulled to ground. The second TIMER
delay begins after the gate of the remote sense switches is
fully ramped up. After the TIMER ramp completes, the PG
pin is activated. An internal circuit pulls-down the TIMER
pin with >100µA of current at all times, except during the
ramping periods, and when VCC is undervoltage.
Calculate the nominal value for the timing capacitor by
inserting the desired delay into Equation 7:
C TIMER =
2µA
• tDLY
1.2V
(7)
For delay times below 60µs, be sure to limit stray capacitances on the TIMER pin by using good PCB design
practices. To program essentially no delay (10mA of current. When all supplies have satisfied
their monitor and overvoltage thresholds, the circuit breaker
has not tripped, the GATE pin has reached its peak, and the
remote sense switches have turned on, a 4µA current
source from VPUMP pulls up PG.
Configure PG as a logic signal by adding an external pullup resistor to a voltage source. For example, create a
negative-logic system reset signal by adding an external
pull-up resistor to the load side of a supply voltage, as in
Figure 8. Calculate the minimum pull-up resistor value that
meets the output low voltage specification for VPG(OL):
RPG(MIN) =
VLO(MAX) − 0.4V
5mA
(10)
Do not pull PG above the GATE pin’s fully ramped voltage.
An internal clamp limits the PG voltage to ≤12.2V relative
to ground. In applications that do not require the PG
output, leave the pin unconnected.
The PG output can also be used as the gate drive for
external N-channel MOSFETs, as in Figure 9. The delay
between the GATE ramp and the PG activation makes a
supply sequencer, useful when two supplies (or two
groups of supplies) need to be ramped one after another.
Choose the FETs and design the ramp rate in the same way
as for the GATE pin. Refer to Equations 5 and 6, substituting 4µA for 10µA, to choose capacitor CPG.
Integrated Switches for Remote Sensing
4µA
PG
ENABLE
Be mindful of thermal effects and power ratings when
choosing a resistor. Place RSENSE as close as possible to
the LTC2921/LTC2922 pins to reduce noise pickup, and
use Kelvin sensing to ensure accurate measurement of the
voltage drop. In applications not requiring the current
sensing circuit breaker, tie the SENSE pin to the VCC pin.
2921/22 F07
A significant feature of the LTC2921/LTC2922 series
is a set of remote sense switches that allow for
compensation of voltage drops in the load path. Switch
activation occurs in the turn-on sequence after the GATE
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VSRC0
RSENSE
VCC
Q0
VL0
RX1
RG0
10Ω
SENSE
4µA
RB1
V1
CGATE
GND
µC
RESET
VS1
CGATE
D1
S1
DC/DC
CONVERTER
PG
ENABLE
GATE
LTC2922
RA1
RZ1
RPG
PG
RG1
10Ω
RY1
VFB
GATE
VPUMP
LOAD
GND
2921/22 F10
GND
LTC2922
VL1
Q1
VSRC1
VOUT
Figure 10. Automatic Remote Sense Switching Connection
2921/22 F08
Figure 8. PG Pin as Logic Output
Q5
VSRC5
VSRC0
RSENSE
VCC
VL5
RG5
10Ω
Q0
VL0
RG0
10Ω
SENSE
GATE
VPUMP
CGATE
To choose a value for resistor RX1, consider the remote
sense switch connection equivalent network in Figure 11.
Resistor RQ1(ON) represents the on-resistance of Q1, and
resistor RFB1(ON) represents the on-resistance of the internal switch.
4µA
RX1
RY1
PG
PG
ENABLE
VFB
RQ1(ON)
IQ1
VSRC1
VOUT
LTC2922
IDS1
S1
VS1
CPG
VL1
RFB1(ON)
D1
RZ1
GND
LTC2922
LOAD
GND
2921/22 F09
DC/DC
CONVERTER
IL1
2921/22 F11
Figure 9. PG Pin as Sequenced N-Channel Gate Driver
Figure 11. Remote Sense Switch Connection Equivalent Network
pin has fully ramped up. The switches are N-channel
MOSFETs whose gates are ramped from ground to VPUMP
at a nominal rate of 8V/ms. The PG pin is activated upon
completion of the TIMER delay cycle that follows GATE
ramp-up and remote sense switch activation. When conditions indicate a supply disconnect, the switches shut off
in less than 10µs.
To allow the load voltage to dominate the feedback to the
converter when the internal switch is closed, make RX1 >>
RFB1(ON). To set the converter feedback ratio accurately
with RY1 and RZ1, make both RX1 and RFB1(ON) much less
than (RY1 + RZ1). To ensure that most of the load current
flows through the external N-channel FET, choose (RX1 +
RFB1(ON)) >> RQ1(ON). Summarized, these requirements
amount to:
Figure 10 shows an example of how to connect a switch to
remote sense the load voltage. Although only one remote
sense switch is referred to in this section, the calculations
and comments apply to all.
Before the activation of Q1 and the internal switch, resistor
RX1 provides a direct path between the DC/DC converter’s
output voltage and its feedback network (RY1 and RZ1).
Once Q1 activates, the supply energizes the load. When
the internal switch turns on, it provides a remote sense
path between the load voltage and the converter’s feedback network.
RQ1(ON), RFB(ON) VCC(MON)(MAX) and VSRC0(MAX) 4.5V of gatesource voltage, even at maximum supply voltage (5.375V)
and minimum GATE pin voltage (10V). Considering the
voltages, temperatures, and currents involved, the maximum on-resistance (RQ(ON)(MAX)) of the Vishay Siliconix
Si2316DS is about 150mΩ. Switches Q1 and Q2 will see
even higher gate-source voltages, implying even smaller
RQ(ON)(MAX) values. Table 2 summarizes the calculated
VQ(ON)(MAX) voltages. Include the additional 50mV drop
across RSENSE when budgeting for the VCC supply path.
Table 2. External MOSFET Drain-Source Voltage Drops
Supply
Voltage
External
MOSFET
RQ(ON)
Max
IL
Max
VQ(ON)
Max
5V
Q0
~150mΩ
0.8A
120mV
(+50mV = 170mV)
3.3V
Q1
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