LTC3770 Datasheet, PDF(13/24 Page) Linear Technology – Synchronous Controller with Margining, Tracking and PLL

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LTC3770 Datasheet, PDF (13/24 Pages) Linear Technology – Synchronous Controller with Margining, Tracking and PLL

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LTC3770

APPLICATIO S I FOR ATIO

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 LTC3770 will

detect this fault condition and provide a minimum ION

current of 5ÂµA to 10ÂµA.

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 induc-

tance, resulting in increased duty cycle as the load current

increases. By lengthening the on-time slightly as current

increases, constant frequency operation can be main-

tained. 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 applica-

tion. 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.

Minimum Off-Time and Dropout Operation

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

time that the LTC3770 is capable of turning on the bottom

MOSFET, tripping the current comparator and turning the

VOUT

RVON1

30k

RVON2

100k

CVON

0.01ÂµF

VON

LTC3770

ITH

(3a)

VOUT

INTVCC

RVON1

RVON2

10k 10k

2N5087

CVON

0.01ÂµF

VON

LTC3770

ITH

3770 F03

(3b)

Figure 3. Correcting Frequency Shift with Load Current Changes

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 4.

2.0

1.5

DROPOUT

REGION

1.0

0.5

0.25

0.50

0.75

1.0

DUTY CYCLE (VOUT/VIN)

3770 F04

Figure 4. 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

VOUT

ââ

ââ1â

VOUT

VIN

â â

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

does not exceed a specified maximum, the inductance

3770f

▷