LT1959_15 Datasheet, PDF (14/24 Pages) Linear Technology – 4.5A, 500kHz Step-Down Switching Regulator

LT1959
APPLICATIONS INFORMATION
R_FB
INPUT
LT1959
IN
R_HI
SHDN
2.38V
3.5µA
C1
R_LO
0.4V
GND
V_SW
LOCKOUT
TOTAL
SHUTDOWN
OUTPUT
+
1959 F04
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:
[ ^{( )} ] ^RLO ^VIN ^{− 2.38 ∆V/ V}OUT ^{+ 1 + ∆V}
R_HI =
( ) ^{2.38 − R2 3.5µA}
( )( ) ^RFB ^{= R}HI ^VOUT ^/∆V
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.
[ ^{( )} ] ^{25k 6 − 2.38 1.5 / 5 + 1 + 1.5}
R_HI =
( ) ^{2.38 − 25k 3.5µA}
( ) ^{25k 5.2}
=
= 48k
2.29
( ) ^RFB ^{= 48k 5 /1.5 = 160k}
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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