Datasheet

14
LT1506
APPLICATIONS INFORMATION
WUU
U
2.38V
0.4V
GND
V
SW
LT1506
INPUT
R
FB
R
HI
1506 F04
OUTPUT
SHDN
LOCKOUT
IN
TOTAL
SHUTDOWN
3.5µA
R
LO
C1
+
Figure 4. Undervoltage Lockout
Keep the connections from the resistors to the shutdown
pin short and make sure that interplane or surface capaci-
tance to the switching nodes are minimized. If high
resistor values are used, the shutdown pin should be
bypassed with a 1000pF capacitor to prevent coupling
problems from the switch node. If hysteresis is desired in
the undervoltage lockout point, a resistor R
FB
can be
added to the output node. Resistor values can be calcu-
lated from:
R
RV VV V
RA
RRV V
HI
LO IN OUT
FB HI OUT
=
−+
()
+
[]
−
()
=
()( )
238 1
238 235
./
..
/
∆∆
∆
µ
25k suggested for R
LO
V
IN
= Input voltage at which switching stops as input
voltage descends to trip level
∆V = Hysteresis in input voltage level
Example: output voltage is 5V, switching is to stop if input
voltage drops below 6V and should not restart unless
input rises back to 7.5V. ∆V is therefore 1.5V and V
IN
= 6V.
Let R
LO
= 25k.
R
k
kA
k
k
Rk k
HI
FB
=
−+
()
+
[]
−
()
=
()
=
=
()
=
25
6
23815 5 1 15
238 25 35
25 5 2
229
48
48 5 1 5 160
../ .
..
.
.
/.
µ
SWITCH NODE CONSIDERATIONS
For maximum efficiency, switch rise and fall times are
made as short as possible. To prevent radiation and high
frequency resonance problems, proper layout of the com-
ponents connected to the switch node is essential. B field
(magnetic) radiation is minimized by keeping catch diode,
switch pin, and input bypass capacitor leads as short as
possible. E field radiation is kept low by minimizing the
length and area of all traces connected to the switch pin
and BOOST pin. A ground plane should always be used
under the switcher circuitry to prevent interplane cou-
pling. A suggested layout for the critical components is
shown in Figure 5. Note that the feedback resistors and
compensation components are kept as far as possible