LTC1778 datasheet(12/24 Pages) LINER

12 / 24 page

LTC1778/LTC1778-1

1778fb

Because the voltage at the ION pin is about 0.7V, the

current into this pin is not exactly inversely proportional to

VIN, especially in applications with lower input voltages.

To correct for this error, an additional resistor RON2

connected from the ION pin to the 5V INTVCC supply will

further stabilize the frequency.

Changes in the load current magnitude will also cause

frequency shift. Parasitic resistance in the MOSFET

switches and inductor reduce the effective voltage across

the inductance, resulting in increased duty cycle as the

load current increases. By lengthening the on-time slightly

as current increases, constant frequency operation can be

maintained. This is accomplished with a resistive divider

from the ITH pin to the VON pin and VOUT. The values

required will depend on the parasitic resistances in the

specific application. A good starting point is to feed about

25% of the voltage change at the ITH pin to the VON pin as

shown in Figure 4a. Place capacitance on the VON pin to

filter out the ITH variations at the switching frequency. The

resistor load on ITH reduces the DC gain of the error amp

and degrades load regulation, which can be avoided by

using the PNP emitter follower of Figure 4b.

Minimum Off-time and Dropout Operation

The minimum off-time tOFF(MIN) is the smallest amount of

time that the LTC1778 is capable of turning on the bottom

MOSFET, tripping the current comparator and turning the

MOSFET back off. This time is generally about 250ns. The

minimum off-time limit imposes a maximum duty cycle of

tON/(tON + tOFF(MIN)). If the maximum duty cycle is reached,

due to a dropping input voltage for example, then the

output will drop out of regulation. The minimum input

voltage to avoid dropout is:

IN MIN

OUT

OFF MIN

()

A plot of maximum duty cycle vs frequency is shown in

Figure 5.

Inductor Selection

Given the desired input and output voltages, the inductor

value and operating frequency determine the ripple

current:

∆ =

⎛

⎝

⎜

⎞

⎠

⎟

−

⎛

⎝

⎜

⎞

⎠

⎟

OUT

Lower ripple current reduces core losses in the inductor,

ESR losses in the output capacitors and output voltage

APPLICATIO S I FOR ATIO

CVON

0.01µF

RVON2

100k

RVON1

30k

VOUT

(4a)

(4b)

VON

ITH

LTC1778

CVON

0.01µF

RVON2

10k

2N5087

RVON1

10k

1778 F04

VOUT

INTVCC

VON

ITH

LTC1778

Figure 4. Correcting Frequency Shift with Load Current Changes

2.0

1.5

1.0

0.5

0.25

0.50

0.75

1778 F05

1.0

DROPOUT

REGION

DUTY CYCLE (VOUT/VIN)

Figure 5. Maximum Switching Frequency vs Duty Cycle

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