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LTC3823

3823fd

The resulting power dissipation in the MOSFETs at maxi-

mum output current are:

PTOP = DTOP IOUT(MAX)2 ρT(TOP) RDS(ON)(MAX)

+ k VIN2 IOUT(MAX) CRSS f

PBOT = DBOT IOUT(MAX)2 ρT(BOT) RDS(ON)(MAX)

Both MOSFETs have I2R losses and the top MOSFET in-

cludes an additional term for transition losses, which are

largest at high input voltages. The constant k = 1.7A–1 can be

used to estimate the amount of transition loss. The bottom

MOSFET losses are greatest when the bottom duty cycle is

near 100%, during a short-circuit or at high input voltage.

Operating Frequency

The choice of operating frequency is a tradeoff between

efﬁciency and component size. Low frequency operation

improves efﬁciency by reducing MOSFET switching losses

but requires larger inductance and/or capacitance in order

to maintain low output ripple voltage.

The operating frequency of LTC3823 applications is de-

termined implicitly by the one-shot timer that controls

the on-time, tON, of the top MOSFET switch. The on-time

is set by the current out of the ION pin and the voltage at

the VON pin according to:

VON

ION

()

Tying a resistor RON to SGND from the ION pin yields an

on-time inversely proportional to 1/3 VIN. The current out

of the ION pin is:

ION

For a step-down converter, this results in approximately

constant frequency operation as the input supply varies:

OUT

VON

•(

)

[]

310

To hold frequency constant during output voltage changes,

tie the VON pin to VOUT. The VON pin has internal clamps

that limit its input to the one-shot timer. If the pin is tied

below 0.6V, the input to the one-shot is clamped at 0.6V.

Similarly, if the pin is tied above 4.8V, the input is clamped

at 4.8V. In high VOUTapplications,tieVONtoINTVCC.Figures

2a and 2b show how RON relates to switching frequency

for several common output voltages.

JUNCTION TEMPERATURE (°C)

–50

1.0

1.5

150

3823 F01

0.5

100

2.0

Figure 1. RDS(ON) vs Temperature

APPLICATIONS INFORMATION

RON (kΩ)

100

1000

3823 F02a

VOUT = 3.3V

VOUT = 1.5V

VOUT = 2.5V

RON (kΩ)

100

1000

100

1000

3823 F02b

VOUT = 3.3V

VOUT = 12V

VOUT = 5V

Figure 2a. Switching Frequency vs RON (VON = 0V)

Figure 2b. Switching Frequency vs RON (VON = INTVCC)

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