lt3689-5 Linear Technology Corporation, lt3689-5 Datasheet - Page 25

• Manufacturer Part Number: lt3689-5
• Description: 700ma Step-down Regulator With Power-on Reset And Watchdog Timer
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
timeout of approximately 25μs. Maximum reset timeout
is limited by the largest available low leakage capacitor.
The accuracy of the timeout period will be affected by
capacitor leakage (the nominal charging current is 2μA)
and capacitor tolerance. A low leakage ceramic capacitor
is recommended.
Selecting the Watchdog Timing Capacitor
The watchdog timeout period is adjustable and can be
optimized for software execution. The watchdog upper
boundary timeout period, t
ing a capacitor, C
Given a specifi ed watchdog timeout period, the capacitor
is determined by:
This equation is accurate for upper boundary periods of
20ms, or greater. The watchdog lower boundary period
(t
The t
In addition, the following equation can be used to calculate
the watchdog lower boundary period for a given C
capacitor value.
These lower boundary period equations are accurate for a
t
periods, see the Watchdog Upper and Lower Boundary
Periods vs Capacitance graphs in the Typical Performance
Characteristics section.
Leaving the C
watchdog timeout of approximately 200μs. Maximum
timeout is limited by the largest available low leakage
capacitor. The accuracy of the timeout period will be af-
fected by capacitor leakage (the nominal charging current
is 2μA) and capacitor tolerance. A low leakage ceramic
capacitor is recommended.
WDL
WDL
C
C
t
WDL
WDT
WDT
WDL
of 3ms, or greater. To program faster t
) has a fi xed relationship to t
=
= t
period is related to t
=
31
t
1
WDT
WDU
WDL
•
t
WDT
pin unconnected will generate a minimum
WDU
• 55
•
. 1 7
, between the C
ms
nF
ms
pF
WDU
WDU
WDU
is adjusted by connect-
by the following:
WDT
for a given capacitor.
pin and ground.
WDU
and t
WDT
WDL
Shorted and Reversed Input Protection
If an inductor is chosen to prevent excessive saturation,
the LT3689 will tolerate a shorted output. When operat-
ing in short-circuit condition, the LT3689 will reduce
its frequency until the valley current is at a typical value
of 1.2A (see Figure 13). There is another situation to
consider in systems where the output will be held high
when the input to the LT3689 is absent. This may occur in
battery charging applications or in battery backup systems
where a battery or some other supply is diode ORed with
Figure 13. The LT3689 Reduces its Frequency to Below
100kHz to Protect Against Shorted Output with 36V Input
Figure 14. Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output; It Also
Protects the Circuit from a Reversed Input. The LT3689
Runs Only When the Input is Present
V
IN
0.5A/DIV
20V/DIV
V
SW
I
L
V
EN/UVLO
IN
LT3689
GND
OUT
LT3689/LT3689-5
10μs/DIV
BST
SW
DA
FB
3689 F13
+
3689 F14
V
OUT
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3689fa