LTC3823IGN datasheet(13/26 Pages) LINER

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LTC3823

3823fd

When there is no RON resistor connected to the ION pin,

the on-time tON is theoretically inﬁnite, which in turn could

damage the converter. To prevent this, the LTC3823 detects

this fault condition and provides a minimum ION current

of 5μA to 10μA.

Changes in the load current magnitude will cause fre-

quency 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 3a. 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 3b.

MOSFET back off. This time is generally about 280ns.

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 4.

APPLICATIONS INFORMATION

CVON

0.01μF

RVON2

100k

RVON1

30k

VOUT

(3a)

(3b)

VON

ITH

LTC3823

CVON

0.01μF

RVON2

10k

2N5087

RVON1

10k

3823 F03

VOUT

INTVCC

VON

ITH

LTC3823

Figure 3. Correcting Frequency Shift with Load Current Changes

Minimum Off-Time and Dropout Operation

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

time that the LTC3823 is capable of turning on the bottom

MOSFET, tripping the current comparator and turning the

2.0

1.5

1.0

0.5

0.25

0.50

0.75

3823 F04

1.0

DROPOUT

REGION

DUTY CYCLE (VOUT/VIN)

Figure 4. Maximum Switching Frequency vs Duty Cycle

Inductor Selection

Given the desired input and output voltages, the induc-

tor value and operating frequency determine the ripple

current:

ΔI

OUT

⎛

⎝

⎜

⎞

⎠

⎟ −

⎛

⎝

⎜

⎞

⎠

⎟

•

Lower ripple current reduces core losses in the inductor,

ESR losses in the output capacitors and output voltage

ripple. Highest efﬁciency operation is obtained at low

frequency with small ripple current. However, achieving

this requires a large inductor. There is a tradeoff between

component size, efﬁciency and operating frequency.

A reasonable starting point is to choose a ripple current

that is about 40% of IOUT(MAX). The largest ripple current

occurs at the highest VIN. To guarantee that ripple current

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