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LT1506

APPLICATIONS INFORMATION

2.38V

0.4V

GND

VSW

LT1506

INPUT

RFB

RHI

1506 F04

OUTPUT

SHDN

LOCKOUT

TOTAL

SHUTDOWN

3.5

µA

RLO

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:

V V

OUT

−+

()+

[]

−

()

()(

)

238

2 35

∆∆

∆

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.

−+

()+

[]

−

()

()=

25 6

238 1 5 5 1

1 5

238 25 3 5

25 5 2

229

48 5 1 5

160

.. /

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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