LTC3711 Datasheet, PDF(12/24 Page) Linear Technology – 5-Bit Adjustable, Wide Operating Range, No RSENSE

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LTC3711 Datasheet, PDF (12/24 Pages) Linear Technology – 5-Bit Adjustable, Wide Operating Range, No RSENSE

◁

LTC3711

APPLICATIO S I FOR ATIO

VOUT

RVON1

30k

RVON2

100k

CVON

0.01ÂµF

VON

LTC3711

ITH

3711 F04a

VOUT

INTVCC

RVON1

RVON2

10k 10k

2N5087

CVON

0.01ÂµF

VON

LTC3711

ITH

3711 F04b

(4a)

(4b)

Figure 4. Correcting Frequency Shift with Load Current Changes

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 LTC3711 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:

VIN(MIN)

VOUT

tON

+ tOFF(MIN)

tON

A plot of maximum duty cycle vs frequency is shown in

Figure 5.

2.0

1.5

DROPOUT

REGION

1.0

0.5

0.25

0.50

0.75

1.0

DUTY CYCLE (VOUT/VIN)

3711 F05

Figure 5. Maximum Switching Frequency vs Duty Cycle

Inductor Selection

Given the desired input and output voltages, the inductor

value and operating frequency determine the ripple

current:

âIL

ï£«

ï£ï£¬

VOUT

ï£¶ï£«

ï£¸ï£· ï£ï£¬1â

VOUT ï£¶

VIN ï£¸ï£·

Lower ripple current reduces cores 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

does not exceed a specified maximum, the inductance

should be chosen according to:

3711f

▷