LTC1436AIGN datasheet(20/28 Pages) LINER

20 / 28 page

LTC1436A

LTC1436-PLL-A/LTC1437A

APPLICATIONS INFORMATION

Figure 11a. 12V Output Auxiliary Regulator Using

Internal Feedback Resistors

The minimum on-time for the LTC1436A/LTC1437A in a

properly configured application is less than 300ns but

increases at low ripple current amplitudes (see Figure 12).

If an application is expected to operate close to the

minimum on-time limit, an inductor value must be chosen

that is low enough to provide sufficient ripple amplitude to

meet the minimum on-time requirement. To determine the

proper value, use the following procedure:

1. Calculate on-time at maximum supply, tON(MIN) =

(1/f)(VOUT/VIN(MAX)).

2. Use Figure 12 to obtain the peak-to-peak inductor ripple

current as a percentage of IMAX necessary to achieve

the calculated tON(MIN).

3. Ripple amplitude

∆IL(MIN) = (% from Figure 12) (IMAX)

where IMAX = 0.1/RSENSE.

4. LMAX = t

ON MIN

IN MAX

OUT

L MIN

()

–

∆













Choose an inductor less than or equal to the calculated

LMAX to ensure proper operation.

1436 F11a

VSEC = 1.19V 1 +

> 13V

()

ON/OFF

VSEC

SECONDARY WINDING

µF

1:N

VOAUX

12V

AUXDR

LTC1436A

LTC1437A

AUXFB

AUXON

SFB

Figure 11c. Auxiliary Comparator Configuration

–

AUXON

AUXFB

ON/OFF

INPUT

VPULL-UP < 8.5V

AUXDR

OUTPUT

1436 F11c

1.19V REFERENCE

LTC1436A

LTC1437A

Figure 11b. 5V Output Auxiliary Regulator Using

External Feedback Resistors

ON/OFF

VSEC

SECONDARY WINDING

1436 F11b

µF

1:N

VOAUX

AUXDR

LTC1436A

LTC1437A

AUXFB

AUXON

VSEC = 1.19V 1 +

()

VOAUX = 1.19V 1 +

()

SFB

Minimum On-Time Considerations

Minimum on-time, tON(MIN), is the smallest amount of

time that the LTC1436A/LTC1437A are capable of turning

the top MOSFET on and off again. It is determined by

internal timing delays and the gate charge required to turn

on the top MOSFET. Low duty cycle applications may

approach this minimum on-time limit. If the duty cycle

falls below what can be accommodated by the minimum

on-time, the LTC1436A/LTC1437A will begin to skip cycles.

The output voltage will continue to be regulated, but the

ripple current and ripple voltage will increase. Therefore

this limit should be avoided.

Figure 12. Minimum On-Time vs Inductor Ripple Current

Because of the sensitivity of the LTC1436A/LTC1437A

current comparator when operating close to the minimum

on-time limit, it is important to prevent stray magnetic flux

generated by the inductor from inducing noise on the

current sense resistor, which may occur when axial type

cores are used. By orienting the sense resistor on the

radial axis of the inductor (see Figure 13), this noise will be

minimized.

INDUCTOR RIPPLE CURRENT (% OF IMAX)

200

250

300

350

400

RECOMMENDED

REGION FOR MIN

ON-TIME AND

MAX EFFICIENCY

1435A F12

LTC1436AIGN Datasheet(PDF) 20 Page - Linear Technology

LTC1436AIGN Datasheet(HTML) 20 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL