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 36V; a 38V OVLO protects the device 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 LT3640 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.
n
n
n
n
n
n
n
n
n
n
High Voltage Buck Regulator:
4V to 36V Operating Range
1.3A Output Current
OVLO Protects Input to 55V
Low Voltage Synchronous Buck Regulator:
2.5V to 5.5V Input Voltage Range
1A Output Current
Synchronizable, Adjustable 350kHz to 2.5MHz
Switching Frequency
Programmable Power-On Reset Timer
Programmable Window Mode Watchdog Timer
Quiescent Current: 275μA
Short-Circuit Robust
Programmable Soft-Start
Low Shutdown Current: I
Q
< 1μA
Available in Thermally Enhanced 28-Lead
(4mm
×
5mm) QFN and 28-Lead TSSOP Packages
APPLICATIONS
n
n
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 3.3V/0.8A and 1.8V/0.8A Step Down Regulators
0.22μF
V
IN
5V TO 34V
10μF
EN/UVLO V
IN
SW
SYNC
PGOOD
WDE
V
OUT1
100k
100k
RST1
RST2
WDO
WDI
CWDT
CPOR
1.5nF
LT3640
V
IN2
EN2
SW2
100k
RT GND SS2
FB2
SS1
1nF
1nF
3640 TA01a
HV Channel Efficiency,
2MHz, V
OUT1
= 3.3V
90
90
LV Channel Efficiency,
2MHz, V
OUT2
= 1.8V
V
IN2
= 3.3V
3.3μH
BST SW1
DA
FB1
80.6k
V
OUT1
3.3V/0.8A
22μF
EFFICIENCY (%)
85
V
IN
= 12V
EFFICIENCY (%)
0.2
0.4
0.6
0.8
1.0
V
OUT1
CURRENT (A)
85
49.9k
80
80
10μF
1μH
V
OUT2
1.8V/0.8A
47μF
μP
75
75
49.9k
70
0
1.2
3640 TA01b
70
0
0.2
0.4
0.6
V
OUT2
CURRENT (A)
0.8
3640 TA01c
1.5nF
32.4k
1
3640p
1
LT3640
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,
PGOOD,
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)
DA Current ..................................................................2A
Operating Junction Temperature Range (Note 2)
LT3640E ................................................. –40°C to 125°C
LT3640I .................................................. –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature, FE Only (Soldering, 10 sec) .... 300°C
PIN CONFIGURATION
TOP VIEW
EN/UVLO
PGOOD
FB2
PGOOD
EN/UVLO
SYNC
SS1
FB1
RT
RST2
RST1
1
2
3
4
5
6
7
8
9
29
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
WDE
WDI
GND
GND
CPOR
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
EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
UFD PACKAGE
28-LEAD (4mm 5mm) PLASTIC QFN
θ
JA
= 34°C/W
EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3640EFE#PBF
LT3640IFE#PBF
LT3640EUFD#PBF
LT3640IUFD#PBF
TAPE AND REEL
LT3640EFE#TRPBF
LT3640IFE#TRPBF
LT3640EUFD#TRPBF
LT3640IUFD#TRPBF
PART MARKING*
LT3640FE
LT3640FE
3640
3640
PACKAGE DESCRIPTION
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 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/
3640p
2
LT3640
ELECTRICAL CHARACTERISTICS
PARAMETER
V
IN
Undervoltage Lockout Threshold
V
IN
Undervoltage Release Threshold
V
IN
Overvoltage Lockout Threshold
V
IN
Overvoltage 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
ΔFB1 to Start LV Channel
ΔFB1 Hysteresis to Stop LV Channel
V
IN2
Minimum Operating Voltage
V
IN2
Maximum Operating Voltage
EN2 Threshold Voltage
FB2 Voltage
FB2 Bias Current
FB2 Line Regulation
SW2 Minimum Off-Time
SW2 PMOS Current Limit
SW2 NMOS Current Limit
SW2 PMOS R
DS(ON)
SW2 NMOS R
DS(ON)
ΔFB2 to Enable
PGOOD
ΔFB2 Hysteresis to Disable
PGOOD
PGOOD
Voltage
FB2 = 0.6V, I
PGOOD
= 1mA
(Note 5)
(Note 5)
I
SW2
= 0.5A (Note 6)
I
SW2
= 0.5A (Note 6)
20
20
l
l
l
l
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
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