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LT1506

APPLICATIONS INFORMATION

and does not fall to less than 1/3 of nominal output voltage,

foldback will not take effect. With the overloaded condi-

tion, output current will increase to a typical value of 5.7A,

determined by peak switch current limit of 6A. With

VIN = 15V, VOUT = 4V (5V overloaded) and IOUT = 5.7A:

D AVG

()=

−

()

5 7 15 4

418

This is safe for short periods of time, but it would be

prudent to check with the diode manufacturer if continu-

ous operation under these conditions must be tolerated.

BOOST PIN CONSIDERATIONS

For most applications, the boost components are a 0.27

µF

capacitor and a 1N914 or 1N4148 diode. The anode is

connected to the regulated output voltage and this gener-

ates a voltage across the boost capacitor nearly identical

to the regulated output. In certain applications, the anode

may instead be connected to the unregulated input volt-

age. This could be necessary if the regulated output

voltage is very low (< 3V) or if the input voltage is less than

5V. Efficiency is not affected by the capacitor value, but the

capacitor should have an ESR of less than 1

Ω to ensure

that it can be recharged fully under the worst-case condi-

tion of minimum input voltage. Almost any type of film or

ceramic capacitor will work fine.

For nearly all applications, a 0.27

µF boost capacitor works

just fine, but for the curious, more details are provided

here. The size of the boost capacitor is determined by

switch drive current requirements. During switch on time,

drain current on the capacitor is approximately IOUT/ 50. At

peak load current of 4.25A, this gives a total drain of 85mA.

Capacitor ripple voltage is equal to the product of on time

and drain current divided by capacitor value;

∆V = (tON)(85mA/C). To keep capacitor ripple voltage to

less than 0.6V (a slightly arbitrary number) at the worst-

case condition of tON = 1.8µs, the capacitor needs to be

0.27

µF. Boost capacitor ripple voltage is not a critical

parameter, but if the minimum voltage across the capaci-

tor drops to less than 3V, the power switch may not

saturate fully and efficiency will drop. An

approximate

formula for absolute minimum capacitor value is:

MIN

OUT

= ()(

)

()

−

()

f = Switching frequency

VOUT = Regulated output voltage

VIN = Minimum input voltage

This formula can yield capacitor values substantially less

than 0.27

µF, but it should be used with caution since it

does not take into account secondary factors such as

capacitor series resistance, capacitance shift with tem-

perature and output overload.

SHUTDOWN FUNCTION AND UNDERVOLTAGE

LOCKOUT

Figure 4 shows how to add undervoltage lockout (UVLO)

to the LT1506. Typically, ULVO is used in situations where

the input supply is

current limited, or has a relatively high

source resistance. A switching regulator draws constant

power from the source, so source current increases as

source voltage drops. This looks like a negative resistance

load to the source and can cause the source to current limit

or latch low under low source voltage conditions. ULVO

prevents the regulator from operating at source voltages

where these problems might occur.

Threshold voltage for lockout is about 2.38V, slightly less

than the internal 2.42V reference voltage. A 3.5

µA bias

current flows

out of the pin at threshold. This internally

generated current is used to force a default high state on

the shutdown pin if the pin is left open. When low shut-

down current is not an issue, the error due to this current

can be minimized by making RLO 10k or less. If shutdown

current is an issue, RLO can be raised to 100k, but the error

due to initial bias current and changes with temperature

should be considered.

()

−

()

−

()

238

3 5

to 100k 25k suggested

.. µ

VIN = Minimum input voltage

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