LT1507CS8 datasheet(15/20 Pages) LINER

15 / 20 page

LT1507

APPLICATIONS INFORMATION

is not a problem, but it should be noted that for

equal case

size, the ripple current rating and ESR of higher voltage

capacitors will be somewhat worse. The lower input

operating voltages of the LT1507 allow considerable

derating of capacitor voltage. If solid tantalum units are

used, it would be wise to use units rated at 25V or more,

as long as ripple current requirements are met. Design

Note 122 discusses the problem of showing typical input

capacitor surges that occur when batteries or adapters are

hot plugged to typical regulator systems.

A new capacitor type known as OS-CON uses a “semicon-

ductor” dielectric to achieve extremely low ESR and high

ripple current rating. These are ideal for input bypassing

because they are not surge sensitive. They are not sug-

gested for output capacitors because the very low ESR

may present loop stability problems. Price and size (height)

are issues to be considered. The original manufacturer is

Sanyo but there are now additional sources.

Larger capacitors may be necessary when the input volt-

age is very close to the minimum specified on the data

sheet. A 5

µF ceramic input capacitor for instance, moves

at about 0.1V/

µs during switch ON time when load current

is 1A, creating a ripple voltage due to reactance. This is in

addition to the ripple caused by capacitor ESR. Physically

larger input capacitors will have more capacitance (less

reactance)

and lower ESR. Small voltage dips during

switch ON time are not normally a problem, but at very low

input voltage they may cause erratic operation because the

input voltage drops below the minimum specification.

Problems can also occur if the input to output voltage

differential is near minimum.

Minimum Input Voltage (After Start-Up)

Minimum input voltage to make the LT1507 “run” cor-

rectly is typically 3.6V, but to regulate the output, a buck

converter input voltage must always be higher than the

output voltage. To calculate minimum operating input

voltage, switch voltage loss and maximum duty cycle

must be taken into account. With the LT1507 there is the

additional consideration of proper operation of the boost

circuit. The boost circuit allows the power switch to

saturate for high efficiency, but it also sometimes results

in a start-up or low current operating voltage that is 0.5V

to 1.5V higher than the standard running voltage, espe-

cially at light loads. An approximate formula to calculate

minimum

running voltage at load currents above 100mA

is:

ImA

IN MIN

OUT

()

()( .

)

()

+Ω

≥

085

100

With VOUT = 3.3V and IOUT = 0.1A, this formula yields

VIN(MIN) = 3.9V. Increasing load current to 1A raises

minimum input to 4.2V. For start-up and operation at light

loads, see the next section.

Minimum Start-Up Voltage and Operation

at Light Loads

The boost capacitor supplies current to the BOOST pin

during switch ON time. This capacitor is recharged only

during switch OFF time. Under certain conditions of light

load and low input voltage, the capacitor may not be fully

recharged during the relatively short OFF time. This causes

the boost voltage to collapse and minimum input voltage

is increased. Start-up voltage at light loads is higher than

normal running voltage for the same reasons. Figure 5

shows minimum input voltage for a 3.3V output, both for

start-up and for normal operation. This graph indicates

that a 5V to 3.3V converter with 4.7V minimum input

voltage, will not start correctly below a 40mA load current

and will not run correctly below a 4mA load current. If

minimum load current is less than 50mA, a preload should

be added or the circuit in Figure 6 can be used.

LOAD CURRENT (mA)

6.5

6.0

5.5

5.0

4.5

4.0

3.5

3.0

100

1000

LT1400 • GXX

VALID ONLY FOR VOUT = 3.3V

MINIMUM VOLTAGE

TO START WITH

STANDARD CIRCUITS

MINIMUM VOLTAGE

TO START WITH

PNP ADDED

MINIMUM VOLTAGE

TO RUN WITH

STANDARD CIRCUIT

MINIMUM VOLTAGE

TO RUN WITH

PNP ADDED

MINIMUM VOLTAGE

TO RUN WITH

PNP ADDED

Figure 5. Minimum Input Voltage for VOUT = 3.3V

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