LTC3901
Secondary Side
Synchronous Driver for Push-Pull
and Full-Bridge Converters
FEATURES
■
■
■
■
■
■
■
■
DESCRIPTIO
N-Channel Synchronous MOSFET Driver
Programmable Timeout
Reverse Inductor Current Sense
Gate Drive Transformer Synchronization
Sequence Monitor
Wide V
CC
Supply Range: 4.5V to 11V
15ns Rise/Fall Times at V
CC
= 5V, C
L
= 4700pF
Undervoltage Lockout
Small 16-Lead SSOP Package
The LTC
®
3901 is a secondary side synchronous rectifier
driver designed to be used in isolated push-pull and full-
bridge converter power supplies. The chip drives two
external N-channel MOSFETs and accepts a transformer-
generated bipolar input to maintain sychronization with
the primary side controller.
The LTC3901 provides a full range of protection features
for the external MOSFETs. A programmable timeout func-
tion is included that disables both drivers when the syn-
chronization signal is missing or incorrect. Additionally,
the chip senses the output inductor current through the
drain-source resistance of the two MOSFETs, turning off
the MOSFETs if the inductor current reverses. The LTC3901
also shuts off the drivers if the supply is low or if the
synchronization sequence is incorrect.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
APPLICATIO S
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48V Input Isolated DC/DC Converters
Isolated Telecom Power Supplies
Distributed Power Step-Down Converters
Industrial Control System Power Supplies
Automotive and Heavy Equipment
TYPICAL APPLICATIO
L1
ISOLATION
BARRIER
T1
V
IN
36V TO 72V
CSE +
MA
MB
ME
ME
CSE –
DRVA
DRVB
LTC3723
PUSH-PULL
CONTROLLER
COMP V
FB
SDRA SDRB
GND
V
CC
LTC3901
+
CSF
PV
CC
MF
MF
CSF –
SYNC
T2
OUT
FB
LT4430 OR LT1431
OPTOCOUPLER
DRIVER
COMP
3901 F01
PGND
TIMER
Figure 1. Simplified Isolated Push-Pull Converter
3901f
U
+
C
OUT
V
OUT
12V
U
U
1
LTC3901
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
PV
CC
1
ME 2
ME 3
PGND 4
CSE
–
5
CSE
+
6
TIMER 7
GND 8
16 V
CC
15 MF
14 MF
13 PGND
12 CSF
–
11 CSF
+
10 GND
9
SYNC
Supply Voltage
V
CC
, PV
CC ............................................................................
12V
Input Voltage
CSE
–
, CSF
–
, TIMER ................. –0.3V to (V
CC
+ 0.3V)
SYNC ...................................................... –12V to 12V
Input Current
CSE
+
, CSF
+ .....................................................................
15mA
Operating Temperature Range (Note 2) ...–40°C to 85°C
Storage Temperature Range ..................–65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC3901EGN
GN PART
MARKING
3901
GN PACKAGE
16-LEAD NARROW PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 130°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL
V
CC
V
UVLO
I
VCC
Timer
V
TMR
I
TMR
t
TMRDIS
V
TMRMAX
I
CS
+
I
CS
–
V
CSMAX
V
CS
SYNC Input
I
SYNC
V
SYNCP
V
SYNCN
Driver Output
R
ONH
R
ONL
I
PK
Driver Pull-Up Resistance
Driver Pull-Down Resistance
Driver Peak Output Current
SYNC Input Current
SYNC Input Positive Threshold
SYNC Positive Input Hysteresis
SYNC Input Negative Threshold
SYNC Negative Input Hysteresis
Timer Threshold Voltage
Timer Input Current
Timer Discharge Time
Timer Pin Clamp Voltage
CS
+
Input Current
CS
–
Input Current
CS
+
Pin Clamp Voltage
Current Sense Threshold Voltage
PARAMETER
Supply Voltage Range
V
CC
Undervoltage Lockout Threshold
V
CC
Undervoltage Lockout Hysteresis
V
CC
Supply Current
The
●
denotes specifications which apply over the full operating
MIN
●
temperature range.
V
CC
= 5V, T
A
= 25
°
C unless otherwise specified. (Note 3)
CONDITIONS
Rising Edge
Rising Edge to Falling Edge
V
SYNC
= 0V
f
SYNC
= 100kHz, C
ME
= C
MF
= 4700pF (Note 4)
●
●
●
●
TYP
5
4.1
0.5
0.5
7
MAX
11
4.5
1
15
10%
–10
120
UNITS
V
V
V
mA
mA
V
µA
ns
V
4.5
–10%
V
CC
/5
–6
40
2.5
V
TMR
= 0V
C
TMR
= 1000pF, R
TMR
= 4.7k
C
TMR
= 1000pF, R
TMR
= 4.7k
V
CS
+ = 0V
V
CS
– = 0V
I
IN
= 5mA, Driver Off
V
CS
– = 0V
(Note 6)
V
SYNC
=
±10V
(Note 7)
●
●
Current Sense (Note 5)
●
●
±1
±1
11
7.5
3
10.5
13.5
18
±10
1.8
–1.0
●
●
●
●
±1
1.0
–1.8
1.4
0.2
–1.4
0.2
0.9
(Note 7)
I
OUT
= –100mA
I
OUT
= 100mA
(Note 7)
●
1.2
1.6
1.2
1.6
0.8
●
2
2
U
µA
µA
V
mV
mV
µA
V
V
V
V
Ω
Ω
A
3901f
W
U
U
W W
W
LTC3901
ELECTRICAL CHARACTERISTICS
SYMBOL
t
d
t
r
, t
f
PARAMETER
SYNC Input to Driver Output Delay
Driver Rise/Fall Time
Switching Characteristics (Note 8)
The
●
denotes specifications which apply over the full operating
MIN
●
temperature range.
V
CC
= 5V, T
A
= 25
°
C unless otherwise specified. (Note 3)
CONDITIONS
C
ME
= C
MF
= 4700pF, V
SYNC
=
±5V
C
ME
= C
MF
= 4700pF, V
SYNC
=
±5V
TYP
60
15
MAX
120
UNITS
ns
ns
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC3901E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design; characterization and correlation
with statistical process controls.
Note 3:
All currents into device pins are positive; all currents out of device
pins are negative. All voltages are referenced to ground unless otherwise
specified.
Note 4:
Supply current in normal operation is dominated by the current
needed to charge and discharge the external MOSFET gates. This current
will vary with supply voltage, switching frequency and the external
MOSFETs used.
Note 5:
Both CSE
+
, CSE
–
and CSF
+
, CSF
–
current sense comparators have
the same performance specifications.
Note 6:
The current sense comparator threshold has a 0.33%/°C
temperature coefficient (TC) to match the TC of the external MOSFET
R
DSON
.
Note 7:
Guaranteed by design, not subject to test.
Note 8:
Rise and fall times are measured using 10% and 90% levels. Delay
times are measured from
±1.4V
at SYNC input to 20%/80% levels at the
driver output.
TYPICAL PERFOR A CE CHARACTERISTICS
Timeout vs V
CC
5.25
5.20
5.15
5.10
T
A
= 25°C
R
TMR
= 51k
C
TMR
= 470pF
5.25
5.20
5.15
5.10
V
CC
= 5V
R
TMR
= 51k
C
TMR
= 470pF
TIMEOUT (µs)
TIMEOUT (µs)
5.05
5.00
4.95
4.90
4.85
4.80
4.75
4
5
6
8
7
V
CC
(V)
9
10
11
5.05
5.00
4.95
4.90
4.85
4.80
4.75
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
TIMEOUT (µs)
Current Sense Threshold vs
Temperature
18
17 V
CC
= 5V, 11V
16
15
14
13
12
11
10
9
8
7
6
5
4
3
0
25
50
75
–50 –25
TEMPERATURE (°C)
18
17
CURRENT SENSE THRESHOLD (mV)
16
15
14
13
12
11
SYNC POSITIVE THRESHOLD (V)
V
CS(MAX)
CLAMP VOLTAGE (V)
U W
3901 G01
Timeout vs Temperature
10
Timeout vs R
TMR
T
A
= 25°C
9
V
CC
= 5V
= 470pF
C
8
TMR
7
6
5
4
3
2
1
0
0
10 20 30 40 50 60 70 80 90 100
R
TMR
(kΩ)
3901 G03
3901 G02
V
CS(MAX)
Clamp Voltage vs CS
+
Input Current
T
A
= 25°C
1.8
1.7
1.6
1.5
1.4
SYNC Positive Threshold vs
Temperature
V
CC
= 11V
V
CC
= 5V
1.3
1.2
1.1
100
125
10
0
5
25
10
15
20
CS
+
INPUT CURRENT (mA)
30
3901 G05
1.0
–50 –25
0
25
50
75
100
125
TEMPERATURE (°C)
3901 G06
3901 G04
3901f
3
LTC3901
TYPICAL PERFOR A CE CHARACTERISTICS
SYNC Negative Threshold vs
Temperature
–1.0
V
CC
= 5V, 11V
SYNC NEGATIVE THRESHOLD (V)
–1.1
PROPAGATION DELAY (µs)
–1.2
–1.3
–1.4
–1.5
–1.6
–1.7
–1.8
–50 –25
0
25
50
75
100
125
110
100
90
80
70
60
50
40
4
5
6
7
8
9
10
11
TEMPERATURE (°C)
3901 G07
PROPAGATION DELAY (µs)
Propagation Delay vs C
LOAD
120
110
PROPAGATION DELAY (µs)
100
90
80
70
60
50
40
1
2
3
4
6
7
C
LOAD
(nF)
5
8
9
10
SYNC TO ME
SYNC TO MF
T
A
= 25°C
V
CC
= 5V
RISE/FALL TIME (ns)
50
45
40
RISE/FALL TIME (ns)
Rise/Fall Time vs Load
Capacitance
50
45
40
RISE/FALL TIME (ns)
35
30
25
20
15
10
5
0
0
1
2
3
4
5 6
C
LOAD
(nF)
7
8
9
10
RISE TIME
FALL TIME
UNDERVOLTAGE LOCKOUT THRESHOLD
VOLTAGE (V)
T
A
= 25°C
V
CC
= 5V
4
U W
3901 G10
Propagation Delay vs V
CC
120
T
A
= 25°C
C
LOAD
= 4.7nF
120
110
100
90
80
70
60
50
Propagation Delay vs
Temperature
V
CC
= 5V
C
LOAD
= 4.7nF
SYNC TO ME
SYNC TO MF
SYNC TO ME
SYNC TO MF
40
–50 –25
0
25
50
75
100
125
V
CC
(V)
3901 G08
TEMPERATURE (°C)
3901 G09
Rise/Fall Time vs V
CC
50
T
A
= 25°C
C
LOAD
= 4.7nF
45
40
35
30
25
20
15
10
5
4
5
6
7
8
9
10
11
Rise/Fall Time vs Temperature
V
CC
= 5V
C
LOAD
= 4.7nF
35
30
25
20
15
10
5
0
V
CC
(V)
3901 G11
RISE TIME
FALL TIME
RISE TIME
FALL TIME
0
–50 –25
0
25
50
75
TEMPERATURE (°C)
100
125
3901 G12
Undervoltage Lockout Threshold
Voltage vs Temperature
4.5
4.4
4.3
4.2
RISING EDGE
4.1
4.0
3.9
3.8
3.7
3.6
3.5
FALLING EDGE
3.4
3.3
3.2
3.1
3.0
0
25
50
75
–50 –25
TEMPERATURE (°C)
100
125
3901 G13
3901 G14
3901f
LTC3901
TYPICAL PERFOR A CE CHARACTERISTICS
V
CC
Supply Current vs
Temperature
20
C
LOAD
= 4.7nF
18
30
T
A
= 25°C
25
SUPPLY CURRENT (mA)
V
CC
= 11V
V
CC
= 11V
20
15
10
V
CC
= 5V
5
0
0
1
2
3
4
5 6
C
LOAD
(nF)
7
8
9
10
V
CC
SUPPLY CURRENT (mA)
16
14
12
10
8
6
4
–50 –25
0
25
PI FU CTIO S
PVCC (Pin 1):
Driver Supply Input. This pin powers the
ME and MF drivers. Bypass this pin to PGND using a 4.7µF
low ESR capacitor in close proximity to the LTC3901. This
pin should be connected to the same supply voltage as the
V
CC
pin.
ME (Pin 2, 3):
Driver Output for ME. This pin drives the
gate of the external N-channel MOSFET, ME.
PGND (Pin 4,13):
Power Ground. Both drivers return to
this pin. Connect PGND to a high current ground node in
close proximity to the sources of ME and MF.
CSE
+
, CSE
–
(Pin 6, 5):
ME Current Sense Differential
Input. Connect CSE
+
through a series resistor to the drain
of ME and CSE
–
through a series resistor to the source of
ME. The LTC3901 monitors the CSE inputs 250ns after ME
goes high. If the inductor current reverses and flows into
ME causing CSE
+
to rise above CSE
–
by more than 10.5mV,
the LTC3901 pulls ME low. See the Current Sense section
for more details on choosing the resistance values for
R
CSE1
to R
CSE3
.
TIMER (Pin 7):
Timer Input. Connect this pin to an external
R-C network to program the timeout period. The LTC3901
resets the timer at every positive and negative transition of
the SYNC input. If the SYNC signal is missing or incorrect,
the LTC3901 pulls both ME and MF low once the TIMER
pin goes above the timeout threshold. See the Timer sec-
tion for more details on programming the timeout period.
GND (Pin 8,10):
Signal Ground. All internal low power
circuitry returns to this pin. To minimize differential ground
currents, connect GND to PGND right at the LTC3901.
SYNC (Pin 9):
Driver Synchronization Input. 0V at this pin
forces both ME and MF high after an initial negative pulse.
A subsequent positive pulse at SYNC input forces ME to
pull low, whereas a negative pulse forces MF to pull low.
The SYNC signal should alternate between positive and
negative pulses. If the SYNC signal is incorrect, the LTC3901
pulls both MF and ME low.
CSF
+
, CSF
–
(Pin 11, 12):
MF Current Sense Differential
Input. Connect CSF
+
through a series resistor to the drain
of MF and CSF
–
through a series resistor to the source of
MF. The LTC3901 monitors the CSF inputs 250ns after MF
goes high. If the inductor current reverses and flows into
MF causing CSF
+
to rise above CSF
–
by more than 10.5mV,
the LTC3901 pulls MF low. See the Current Sense section
for more details on choosing the resistance values for
R
CSF1
to R
CSF3
.
MF (Pin 14, 15):
Driver Output for MF. This pin drives the
gate of the external N-channel MOSFET, MF.
V
CC
(Pin 16):
Power Supply Input. All internal circuits
except the drivers are powered from this pin. Bypass this
pin to GND using a 1µF capacitor in close proximity to the
LTC3901.
3901f
U W
V
CC
= 5V
V
CC
Supply Current
vs Load Capacitance
50
75
100
125
TEMPERATURE (°C)
3901 G15
3901 G16
U
U
U
5
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