LT1316CS8 datasheet(7/16 Pages) LINER

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LT1316

APPLICATIONS INFORMATION

then adding in the amount of overshoot that will occur due

to turn-off delay of the power transistor. This turn-off

delay is approximately 300ns.

Peak switch current = DC current limit from graph +

VIN/L(turn-off delay)

Example:

Set peak switch current to 100mA for: VIN = 2V,

L = 33

µH

Overshoot = VIN/L(turn-off delay) = (2/33µH)(300ns)

= 18.2mA

Refer to RSET graph and locate

(100mA – 18.2mA)

≈ 82mA

RSET ≈ 33k

Calculating Duty Cycle

For a boost converter running in continuous conduction

mode, duty cycle is constrained by VIN and VOUT according

to the equation:

DC =

VOUT – VIN + VD

VOUT – VSAT + VD

where VD = diode voltage drop ≈ 0.4V and VSAT = switch

saturation voltage

≈ 0.2V.

If the duty cycle exceeds the LT1316’s minimum specified

duty cycle of 0.73, the converter cannot operate in con-

tinuous conduction mode and must be designed for

discontinuous mode operation.

Inductor Selection and Peak Current Limit for

Continuous Conduction Mode

Peak current and inductance determine available output

power. Both must be chosen properly. If peak current or

inductance is increased, output power increases. Once

output power or current and duty cycle are known, peak

current can be set by the following equation, assuming

continuous mode operation:

IPEAK =

2(IOUT)

1 – DC

(1)

Inductance can now be calculated using the peak current:

L =

(tOFF)

VOUT – VIN + VD

0.4(IPEAK)

(2)

where tOFF = 2µs and VD = 0.4V.

As a result of equations 1 and 2, ripple current during

switching will be 40% of the peak current (see Figure 2).

Using these equations at the specified IOUT, the part is

delivering approximately 60% of its maximum output

power. In other words, the part is operating on a 40%

reserve. This is a safe margin to use and can be decreased

if input voltage and output current are tightly controlled.

For some applications, this recommended inductor size

may be too large. Inductance can be reduced but available

output power will decrease. Also, ripple current during

switching will increase and may cause discontinuous

operation. Discontinuous operation occurs when

inductor current ramps down to zero at the end of each

switch cycle (see Figure 4). Shown in Figure 5 is minimum

inductance vs peak current for the part to remain in

continuous mode.

Figure 5. Minimum Inductance vs Peak Current

for Continuous Mode Operation

PEAK CURRENT (mA)

100

1000

100

1000

1316 F05

5V TO 18V

5V TO 12V

2V TO 5V

µs/DIV

Figure 4. Discontinuous Mode Operation

1316 F04

SW PIN

5V/DIV

0mA

INDUCTOR

CURRENT

100mA/DIV

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