LT3689/LT3689-5
700mA Step-Down
Regulator with Power-On Reset
and Watchdog Timer
FEATURES
DESCRIPTION
TheLT®3689isanadjustablefrequency(350kHzto2.2MHz)
monolithicstep-downswitchingregulatorwithapower-on
reset and watchdog timer. The regulator operates from
inputs up to 36V and withstands transientsup to60V. Low
ripple Burst Mode® operation maintains high efficiency at
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Wide Input Range:
Operation from 3.6V to 36V
Overvoltage Lockout Protects Circuits through
60V Transients
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85µA I at 12V to 3.3V
Q
IN
OUT
Low Ripple Burst Mode® Operation Allows Output
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lowoutputcurrentwhilekeepingoutputripplebelow15mV
in a typical application, with input quiescent current of just
85µA. Shutdown circuitry reduces input supply current to
less than 1µA while EN/UVLO is pulled low. Using a resis-
tor divider on the EN/UVLO pin provides a programmable
undervoltage lockout. Current limit, frequency foldback
and thermal shutdown provide fault protection.
Ripple <15mV
P-P
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Programmable, Defeatable Watchdog Timer with
Window or Timeout Control
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Programmable Power-On Reset Timer (POR)
Synchronizable, Adjustable 350kHz to 2.2MHz
Switching Frequency
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700mA Output Switching Regulator with Internal
Power Switch
Fixed 5V or Adjustable Output Voltage
800mV Feedback Voltage
Programmable Input Undervoltage Lockout with
Hysteresis
16-Pin 3mm × 3mm QFN and 16-Pin MSOP Packages
The reset and watchdog timeout periods are indepen-
dentlyadjustableusingexternalcapacitors.Tightaccuracy
specifications and glitch immunity ensure reliable reset
operation of a system without false triggering. The open
collector RST will pull down if output voltage drops 10%
below the programmed value. The watchdog timer is
pin-selectable for window or timeout modes. In timeout
mode, WDO pulls low if too long of a period passes before
a watchdog transition is detected. In window mode, the
LT3689 monitors for WDI falling edges grouped too close
together or too far apart.
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APPLICATIONS
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Automotive Electronic Control Units
Industrial Power Supplies
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L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of
Linear Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
Efficiency
3.3V Regulator with Power-On Reset Timer and Watchdog Timer
90
80
70
60
50
40
30
20
10
0
1000
100
10
V
V
= 12V
IN
OUT
V
IN
= 3.3V
4.5V TO 36V
f = 700kHz
L = 12µH
2.2µF
V
EN/UVLO
LT3689
TRANSIENT TO 60V
OUT
BST
IN
T
= 25°C
A
0.1µF
12µH
10pF
µP
3.3V
I/O
WDI
WDO
RST
SW
700mA
I/O
RESET
DA
FB
RT
316k
C
C
WDT
POR
C
WDT
10nF
= 182ms
= 5.9ms
22µF
C
GND
SYNC
POR
t
20.5k 100k
WDU
WDL
68nF
t
f
= 700kHz
t
= 157ms
SW
RST
1
3689 TA01
0.0001
0.001
0.01
0.1
1
LOAD CURRENT (A)
3689 TA01b
3689fe
1
For more information www.linear.com/LT3689