3641 is a dual channel, current mode monolithic buck
switching regulator with a power-on reset and a watchdog
timer. Both regulators are synchronized to a single oscillator
with an adjustable frequency (350kHz to 2.5MHz). At light
loads, both regulators operate in low ripple Burst Mode
®
to maintain high efficiency and low output ripple.
The high voltage channel is a nonsynchronous buck with
an internal 2.4A top switch that operates from an input
of 4V to 42V and input transient protection to 55V. The
low voltage channel operates from an input of 2.5V to
5.5V. Internal synchronous power switches provide high
efficiency without the need of external Schottky diode.
Both channels have cycle-by-cycle current limit, providing
protection against shorted outputs.
The power-on reset and watchdog timeout periods are
both adjustable using external capacitors. The window
mode watchdog timer flags when the μP pulses group
too close together or too far apart.
The LT3641 is available in a 28-pin 4mm
×
5mm QFN
package and 28-pin TSSOP package. Both packages have
an exposed pad for low thermal resistance.
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High Voltage Buck Regulator:
4V to 42V Operating Range
1.3A Output Current
Input Transient Protection to 55V
Low Voltage Synchronous Buck Regulator:
2.5V to 5.5V Input Voltage Range
1.1A Output Current
Synchronizable, Adjustable 350kHz to 2.5MHz
Switching Frequency
Programmable Power-On Reset Timer
Programmable Window Mode Watchdog Timer
Typical Quiescent Current: 290μA
Short-Circuit Robust
Programmable Soft-Start
Low Shutdown Current: I
Q
< 1μA
Thermal Shutdown
Available in Thermally Enhanced 28-Lead
(4mm
×
5mm) QFN and 28-Lead TSSOP Packages
APPLICATIONS
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Industrial Power Supplies
Automotive Electronic Control Units
L,
LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks
and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
TYPICAL APPLICATION
2MHz 5V/0.8A and 3.3V/0.5A Step Down Regulators
0.22μF
V
IN
7V TO 42V*
10μF
EN/UVLO V
IN
SW
SYNC
WDE
PGOOD2
V
OUT1
100k
100k
RST1
RST2
WDO
WDI
CWDT
CPOR
1.5nF
LT3641
V
IN2
EN2
SW2
226k
RT GND SS2
FB2
SS1
1nF
1nF
3641 TA01a
4.7μH
BST SW1
DA
FB1
301k
HV Channel Efficiency,
2MHz, V
OUT1
= 5V
V
OUT1
5V/0.8A
22μF
85
EFFICIENCY (%)
EFFICIENCY (%)
90
V
IN
= 12V
95
LV Channel Efficiency,
2MHz, V
OUT2
= 3.3V
90
V
IN2
= 5V
100k
85
80
μP
2.2μH
V
OUT2
3.3V/0.5A
22μF
80
75
75
49.9k
1.5nF
32.4k
70
0
0.2
0.4 0.6 0.8 1.0
V
OUT1
CURRENT (A)
1.2
1.4
70
0
0.2
0.4
0.6
0.8
V
OUT2
CURRENT (A)
1.0
1.2
3641 TA01b
3641 TA01c
* FOR INPUT VOLTAGES ABOVE 42V RESTRICTIONS APPLY
3641fa
1
LT3641
ABSOLUTE MAXIMUM RATINGS
(Note 1)
V
IN
, EN/UVLO Voltage (Note 7) .................................55V
WDE
Voltage .............................................................30V
BST Above SW, SW1 Voltage ....................... –0.3V to 6V
SW1 Above SW Voltage ............................... –0.3V to 6V
V
IN2
, SYNC, EN2,
PGOOD2,
WDI,
WDO, RST1, RST2,
Voltages ....................... –0.3V to 6V
SS1, SS2, FB1, FB2, RT, CWDT,
CPOR Voltages………... ........................... –0.3V to 2.5V
SW2 Voltage ................................ –0.3V to (V
IN2
+ 0.3V)
Operating Junction Temperature Range (Note 2)
LT3641E ................................................. –40°C to 125°C
LT3641I .................................................. –40°C to 125°C
LT3641H................................................. –40°C to 150°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature, FE Only (Soldering, 10 sec) .... 300°C
PIN CONFIGURATION
TOP VIEW
EN/UVLO
PGOOD2
FB2
PGOOD2
EN/UVLO
SYNC
SS1
FB1
RT
RST2
RST1
1
2
3
4
5
6
7
8
9
29
GND
28 SS2
27 EN2
26 GND
25 SW2
24 V
IN2
23 GND
22 V
IN
21 BST
20 SW
19 SW1
18 DA
17 NC
16 GND
15 GND
SYNC 1
SS1 2
FB1 3
RT 4
RST2
5
RST1
6
WDO
7
CWDT 8
9 10 11 12 13 14
CPOR
WDE
WDI
GND
GND
NC
29
GND
TOP VIEW
GND
22 SW2
21 V
IN2
20 GND
19 V
IN
18 BST
17 SW
16 SW1
15 DA
SS2
EN2
FB2
28 27 26 25 24 23
WDO
10
CWDT 11
CPOR 12
WDE
13
WDI 14
FE PACKAGE
28-LEAD PLASTIC TSSOP
θ
JA
= 30°C/W,
θ
JC
= 8°C/W
EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
UFD PACKAGE
28-LEAD (4mm
×
5mm) PLASTIC QFN
θ
JA
= 43°C/W,
θ
JC
= 3.4°C/W
EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3641EFE#PBF
LT3641IFE#PBF
LT3641HFE#PBF
LT3641EUFD#PBF
LT3641IUFD#PBF
TAPE AND REEL
LT3641EFE#TRPBF
LT3641IFE#TRPBF
LT3641HFE#TRPBF
LT3641EUFD#TRPBF
LT3641IUFD#TRPBF
PART MARKING*
LT3641FE
LT3641FE
LT3641FE
3641
3641
PACKAGE DESCRIPTION
28-Lead Plastic TSSOP
28-Lead Plastic TSSOP
28-Lead Plastic TSSOP
28-Lead (4mm
×
5mm) Plastic QFN
28-Lead (4mm
×
5mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
–40°C to 125°C
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
3641fa
2
LT3641
ELECTRICAL CHARACTERISTICS
PARAMETER
V
IN
Undervoltage Lockout Threshold
V
IN
Undervoltage Release Threshold
Quiescent Current from V
IN
EN/UVLO Threshold Voltage
EN/UVLO High Bias Current
EN/UVLO Low Bias Current
SYNC Input Frequency
SYNC Threshold Voltage
Switching Frequency
FB1 Voltage
FB1 Bias Current
FB1 Line Regulation
SW1 Minimum Off-Time
SW1 V
CESAT
SW1 Leakage Current
SW1 Current Limit
DA Current limit
BST Pin Current
Minimum BST-SW Voltage
V
IN2
Minimum Operating Voltage
V
IN2
Maximum Operating Voltage
EN2 Threshold
EN2 Hysteresis
EN2 Bias Current
FB2 Voltage
FB2 Bias Current
FB2 Line Regulation
SW2 PMOS Current Limit
SW2 NMOS Current Limit
SW2 PMOS R
DS(ON)
SW2 NMOS R
DS(ON)
ΔFB2 to Enable
PGOOD2
ΔFB2 Hysteresis to Disable
PGOOD2
PGOOD2
Voltage
FB2 = 0.6V, I
PGOOD2
= 1mA
FB2 = 0.6V
2.5V < V
IN2
< 5.5V
(Note 5)
(Note 5)
I
SW2
= 0.5A (Note 6)
I
SW2
= 0.5A (Note 6)
20
20
l
l
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l
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
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