LT1950 Linear Technology, LT1950 Datasheet - Page 11

• Manufacturer Part Number: LT1950
• Description: Single Switch PWM Controller
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
Shutdown and Undervoltage Lockout
Figure 4 shows how to program undervoltage lockout
(UVLO) for the V
situations where the input supply is current limited, or has
a relatively high source resistance. A switching regulator
draws constant power from the source, hence source
current increases as source voltage drops. This looks like
a negative load resistance to the source and can cause the
source to current limit or latch low under low source
voltage conditions. An internally set undervoltage lockout
(UVLO) threshold prevents the regulator from operating at
source voltages where these problems might occur. An
internal comparator will force the part into shutdown
below the minimum V
to prevent excessive discharge of battery-operated sys-
tems. Alternatively, UVLO threshold is adjustable. The
shutdown threshold voltage of the SHDN pin is 1.32V.
Forcing the SHDN pin below this 1.32V threshold causes
V
If the SHDN pin is left open circuit, a permanent 3 A flows
out of the pin to ensure that the pin defaults high to allow
normal operation. Voltages above the 1.32V threshold
cause an extra 7 A to be sourced out of the pin, providing
current hysteresis. This can be used to set voltage hyster-
esis of the UVLO threshold using the following equations:
REF
V
V
R
R
H
L
1
2
to be disabled and stops switching at the GATE pin.
= Turn off threshold
= Turn on threshold
V
(
H
V
7
H
–
A
– .
V
R
L
1 32
1
1 32
.
IN
V
U
V
supply. Typically, UVLO is used in
)
IN
of 2.6V. This feature can be used
3
U
A
W
U
Example: switching should not start until the input is
above 11V and is to stop if the input falls below 9V.
Keep the connections from the resistors to the SHDN pin
short and make sure that the interplane or surface capaci-
tance to the switching nodes are minimized. If high resis-
tance values are used, the SHDN pin should be bypassed
with a 1nF capacitor to prevent coupling problems from
the switch node.
Generating V
The LT1950’s BOOST pin is used to provide a “boosted”
11V supply at the V
Since V
the IC, it is advantageous to generate a boosted V
architecture achieves high GATE drive for an external
C1
V
V
R
R
H
L
V
1
2
IN
= 9V
= 11V
IN2
11
(
11
V
7
R1
R2
supplies the output driver for the GATE pin of
V
–
286
A
IN2
– .
9
LT1950
Figure 4. Undervoltage Lockout
V
1 32
1 32
k
.
Supply Using BOOST Pin
3 A
IN2
286
V
V
)
pin for V
k
3
A
1.32V
7 A
GND
IN
36
voltages as low as 3V.
k
LT1950
1950 F04
sn1950 1950fs
IN2
V
11
REF
+
. This