LTC3778 Просмотр технического описания (PDF) - Linear Technology

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LTC3778

Wide Operating Range, No RSENSE™ Step-Down Controller

Linear Technology 

LTC3778

APPLICATIO S I FOR ATIO

The ρT term is a normalization factor (unity at 25°C)

accounting for the significant variation in on-resistance

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.

2.0

1.5

1.0

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 LTC3778 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 according to:

0.5

0

– 50

0

50

100

150

JUNCTION TEMPERATURE (°C)

3778 F02

Figure 2. RDS(ON) vs. Temperature

The power dissipated by the top and bottom MOSFETs

strongly depends upon their respective duty cycles and

the load current. When the LTC3778 is operating in

continuous mode, the duty cycles for the MOSFETs are:

DTOP

=

VOUT

VIN

DBOT

=

VIN

– VOUT

VIN

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.

tON

=

VVON

IION

(10pF)

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:

f=

VOUT

[Hz]

(VVON) RON(10pF)

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

Similarly, if the pin is tied above 2.4V, the input is clamped

at 2.4V. If output is above 2.4V, use a resistive divider from

VOUT to VON pin.

Because the voltage at the ION pin is about 0.7V, the

current into this pin is not exactly inversely proportional to

VIN, especially in applications with lower input voltages.

To correct for this error, an additional resistor RON2

connected from the ION pin to the 5V INTVCC supply will

further stabilize the frequency.

RON2

=

5V

0.7V

RON

Changes in the load current magnitude will also cause

frequency shift. Parasitic resistance in the MOSFET

switches and inductor reduce the effective voltage across

the inductance, resulting in increased duty cycle as the

3778f

11

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