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LT1959 Datasheet(PDF) 14 Page - Linear Technology

Part No. LT1959
Description  4.5A, 500kHz Step-Down Switching Regulator
Download  24 Pages
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT1959 Datasheet(HTML) 14 Page - Linear Technology

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14
LT1959
APPLICATIONS INFORMATION
2.38V
0.4V
GND
VSW
LT1959
INPUT
RFB
RHI
1959 F04
OUTPUT
SHDN
LOCKOUT
IN
TOTAL
SHUTDOWN
3.5
µA
RLO
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 RFB can be
added to the output node. Resistor values can be calcu-
lated from:
R
RV
V V
V
RA
RR
V
V
HI
LO
IN
OUT
FB
HI
OUT
=
−+
()+
[]
()
=
()(
)
238
1
238
2 35
./
..
/
∆∆
µ
25k suggested for RLO
VIN = 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 VIN = 6V.
Let RLO = 25k.
R
k
kA
k
k
Rk
k
HI
FB
=
−+
()+
[]
()
=
()
=
=
()=
25 6
238 1 5 5 1
1 5
238 25 3 5
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


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