LTC1700 datasheet(13/16 Pages) LINER

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LTC1700

where:

VD = Voltage drop of P-channel parasitic diode

IOUT = Initial load current during start-up

COUT = Output capacitance

Hence you would select the start-up capacitor, CSS, to

ensure tDELAY > tPOWERUP. Remember that the above

equation is only valid for VIN < 2.3V. If VIN is greater than

2.3V, then tPOWERUP = 0ns.

Design Example

Assume the LTC1700 is used to convert a 3.3V input to 5V

output. Load current requirement is a maximum 3A and a

minimum of 100mA. Efficiency at both low and high load

currents is important. Ambient temperature = 25

°C.

Since low load current efficiency is important, Burst Mode

operation is enabled by connecting pin 5 to VOUT.

Duty Cycle = 1 – VIN/VOUT = 0.34

Since the duty cycle is less than 36%, the value of the

inductor is chosen based on the LMINBURST equation.

LMINBURST = 0.8µH.

In the application, (Figure 7) a 4.6

µH inductor is used to

further reduce ripple current. The actual ripple current is

now:

∆IV

kHz









034

530

4 6

046

)

For the main N-channel MOSFET, the RDS(ON) should be:

DS ON N CHANNEL

O MAX

()(

)

()

–

)

−

+∆

=Ω

13 2

Accounting for the peak current reduction due to slope

compensation (see Figure 5), the RDS(ON) of the N-channel

should be:

RDS(ON) = (13.2)(0.9)

= 11.9m

Ω

The factor, 0.9, is obtained from Figure 5 using a duty cycle

of 34%. The peak current of the inductor is 5A. Select an

inductor that does not saturate at this current level. The

average current through the N-channel MOSFET is 1.62A

while the average current through the synchronous P-

channel MOSFET is 3A.

The FDS6670A and FDS6375 are chosen for the

N-channel and P-channel MOSFET respectively. We can

now calculate the temperature rise in the FDS6670A. RMS

current flowing through the FDS6670A is 2.78A. Hence

power dissipated is:

PDISS = (2.78)2 (8 × 10–3)

= 61.82mW

The

θJA of the FDS6670A is 50°C/W. Therefore tempera-

ture rise is:

TRISE = 61.82mW × 50

= 3.1

°C

This is an insignificant temperature rise and therefore the

omission of the

ρT in calculating the required RDS(ON)

does not generate a large error.

At 3A load, the RMS current into the output capacitor is

given by:

ICOUT(RMS) = 3(5/3.3 – 1)0.5 = 2.15A

To meet the RMS current requirement, two SANYO POSCAP

100

µF capacitors are paralleled. These capacitors have

low ESR (55m

Ω) and to futher reduce the overall ESR, a

µF ceramic capacitor is placed in parallel with the

POSCAP capacitor. Figure 7 shows the complete circuit.

APPLICATIONS INFORMATION

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