LTC3704 View Datasheet(PDF) - Linear Technology

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LTC3704

Wide Input Range, No RSENSE™ Positive-to-Negative DC/DC Controller

Linear Technology 

LTC3704

APPLICATIO S I FOR ATIO

The RTH(JA) to be used in this equation normally includes

the RTH(JC) for the device plus the thermal resistance from

the case to the ambient temperature (RTH(CA)). This value

of TJ can then be compared to the original, assumed value

used in the iterative calculation process.

Output Diode Selection

To maximize efficiency, a fast switching diode with low

forward drop and low reverse leakage is desired. The

output diode in a positive-to-negative converter conducts

current during the switch off-time. The peak reverse

voltage that the diode must withstand is equal to VIN(MAX)

– VO. The average forward current in normal operation is

equal to the output current, and the peak current is equal

to the peak inductor current.

ID(PEAK)

=

⎛

–⎝⎜1+

χ⎞

2 ⎠⎟

• IO(MAX)

1–

1

DMAX

The power dissipated by the diode is:

PD = IO(MAX) • VD

and the diode junction temperature is:

TJ = TA + PD • RTH(JA)

The RTH(JA) to be used in this equation normally includes

the RTH(JC) for the device plus the thermal resistance from

the board to the ambient temperature in the enclosure.

Remember to keep the diode lead lengths short and to

observe proper switch-node layout (see Board Layout

Checklist) to avoid excessive ringing and increased

dissipation.

Selecting the DC Coupling Capacitor

The voltage on the coupling capacitor in a positive-to-

negative converter is VIN(MAX) – VO, plus any additional ΔV

due to the ripple currents in the inductors. Generally, the

DC coupling capacitor is dimensioned based on the high

RMS ripple which flows in it, as shown in Figure 13.

The minimum RMS current rating of this capacitor must

exceed:

IRMS(CAP) = –IO(MAX) •

DMAX

1– DMAX

1A/DIV

500ns/DIV

3704 F13

Figure 13. Ripple Current in the DC Coupling Capacitor

A low ESR and ESL, X5R- or X7R-type ceramic capacitor

is recommended here.

Selecting the Output Capacitor

The output ripple voltage appears as a triangular wave-

form riding on VO, due to the ripple current of L2 (the DC

component of the current in L2 equals the output current).

This ripple current flows through the ESR and bulk capaci-

tance of the output capacitor to produce the overall ripple

voltage on this node. Using the off-time to calculate this

ripple current results in the following equation for ΔIL2:

ΔIL2

=

– 1– DMAX

f

•

VO

L2

where VO is a negative number. The output ripple voltage

is therefore:

ΔVO(P–P)

=

1–

DMAX

f

•

VO

L2

⎡

⎢–ESR

⎣

–

8

•

1

f •CO

⎤

⎥

⎦

The ESR can be minimized by using high quality, X5R- or

X7R-dielectric ceramic capacitor in parallel with a larger

value tantalum or aluminum electrolytic bulk capacitor.

Depending upon the application, it may be that the ceramic

capacitor alone will be sufficient.

The RMS ripple current rating of the output capacitor

needs to be greater than:

3704fb

17

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