LTC1778 datasheet(11/24 Pages) LINER

11 / 24 page

LTC1778/LTC1778-1

1778fb

APPLICATIO S I FOR ATIO

with temperature, typically about 0.4%/°C as shown in

Figure 2. For a maximum junction temperature of 100°C,

using a value ρT = 1.3 is reasonable.

The power dissipated by the top and bottom MOSFETs

strongly depends upon their respective duty cycles and

the load current. When the LTC1778 is operating in

continuous mode, the duty cycles for the MOSFETs are:

TOP

OUT

BOT

OUT

–

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

includes 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 LTC1778 applications is deter-

mined 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 into the ION pin and the voltage at the VON

pin (LTC1778-1) according to:

VON

ION

()

VON defaults to 0.7V in the LTC1778.

Tying a resistor RON from VIN to the ION pin yields an on-

time inversely proportional to VIN. For a step-down con-

verter, this results in approximately constant frequency

operation as the input supply varies:

OUT

VON

()

[]

To hold frequency constant during output voltage changes,

tie the VON pin to VOUT or to a resistive divider from VOUT

when VOUT > 2.4V. The VON pin has internal clamps that

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

0.7V, the input to the one-shot is clamped at 0.7V. Simi-

larly, if the pin is tied above 2.4V, the input is clamped at

2.4V. In high VOUT applications, tying VON to INTVCC so

that the comparator input is 2.4V results in a lower value

for RON. Figures 3a and 3b show how RON relates to

switching frequency for several common output voltages.

RON (kΩ)

100

1000

10000

1778 F03a

VOUT = 3.3V

VOUT = 1.5V

VOUT = 2.5V

RON (kΩ)

100

1000

10000

1778 F03b

VOUT = 3.3V

VOUT = 12V

VOUT = 5V

Figure 3a. Switching Frequency vs RON

for the LTC1778 and LTC1778-1 (VON = 0V)

Figure 3b. Switching Frequency vs RON

for the LTC1778-1 (VON = INTVCC)

LTC1778 Datasheet(PDF) 11 Page - Linear Technology

LTC1778 Datasheet(HTML) 11 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL