LT1680ISW datasheet(14/16 Pages) LINER

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LT1680

APPLICATIONS INFORMATION

Setting the resistor divider reference voltage to 2V assures

that the additional compensation waveform will be en-

abled at a 75% duty cycle. As shown in Figure 11a, using

RSL1 = 45k and RSL2 = 30k sets the desired reference

voltage and has a RTH of 18k, which meets both design

requirements. Figure 11b shows the slope compensation

effective waveforms both with and without the SL/ADJ

external resistors.

In a typical LT1680 boost converter, the switch current is

equal to the inductor current, but is chopped according to

duty cycle (DC). The conduction loss (PLOSS) for a given

FET RDS(ON) can be calculated using the relation:

PLOSS ≈ (DC)(RDS(ON))(IAVG2 + [∆I2/12])

where IAVG = average inductor current and ∆I = peak-to-

peak inductor ripple current.

The output diode is often a major source of power loss in

switching regulators and selection of adequately rated

diodes is important. In a boost converter, when the output

voltage is significantly higher than the input voltage, the

peak diode current becomes much higher than average

output currents and diode current ratings must be ob-

served with caution. The peak diode current is:

ID(PEAK) = IAVG + ∆I/2

and the average power dissipation (PD) in the diode is:

PD = (IOUT)(Vf)

where Vf is the forward voltage of the diode at peak

current. The output diode must also be rated for maximum

reverse voltages exceeding VOUT.

CIN and COUT Supply Decoupling Capacitor Selection

The large currents typical of LT1680 applications require

special consideration for the regulator input and output

supply decoupling capacitors.

Under normal steady state boost operation, output current

provided by the converter is a square wave of duty cycle VIN/

VOUT, the average value being equal to the required DC load

current (IOUT). The continuity of the load current is main-

tained by the output bypass capacitors. To prevent exces-

sive output voltage ripple and undue capacitor heating (and

associated catastrophic failure), low ESR output capacitors

sized for the maximum RMS current must be used. This

maximum capacitor RMS current follows the relation:

RMS

OUT

≈









–

Capacitor ripple current ratings are often based on only 2000

hours (3 months) lifetime; it is advisable to derate either the

ESR or temperature rating of capacitors for increased MTBF.

Figure 11a. External Slope Compensation Resistors

RSL1

45k

RSL2

30k

1680 F11a

5VREF

SL/ADJ

LT1680

(0.084 + 0.139)(fO)

RSENSE

(0.084)(fO)

RSENSE

2.5V

0.8V

DC = 0.75

1680 F11b

Figure 11b. Slope Compensation Waveforms

Power MOSFET and Output Rectifying Diode Selection

LT1680 converter system parameters that dictate selec-

tion criteria for the switch MOSFET and output rectifying

diode include maximum load current (IOUT), inductor

average current (IAVG) and inductor ripple current (∆I),

and maximum input and output voltages.

The switch MOSFETs selected must have a maximum

operating VDSS exceeding the maximum output voltage

(VOUT). VGS rated operating maximums must exceed the

12VIN supply voltage. Once voltage requirements have

been determined, switch conduction resistance (RDS(ON))

can be determined based on allowable power dissipation.

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