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LTC1775

paid to the resulting thermal issues. Under DC conditions,

the maximum power that can be dissipated by a MOSFET

switch limits the current through it:

DS MAX

DS ON

J MAX

JA DS ON

TJ MAX

()

(

)

–

θρ

For example, the SUD50N03-10 with TJ(MAX) = 175°C,

TA =70°, θJA = 30° C/W, RDS(ON) = 0.019Ω, ρTJ(MAX) = 1.8

can operate with a maximum DC current of 10A. In a

switching application, the actual power dissipation is

increased by the transition losses and is reduced by the

switch duty cycle. When the LTC1775 is operating in

continuous mode, the duty cycles for the MOSFETs are:

Top Duty Cycle

Bottom Duty Cycle

OUT

–

The MOSFET power dissipations at maximum output

current are:

TOP

OUT

O MAX

T TOP

DS ON

O MAX

RSS

BOT

OUT

O MAX

T BOT

DS ON

















()(

)(

)

()(

)(

)( )

–

()(

)(

)

()

(

)

(

)

()

(

)

(

)

Both MOSFETs have I2R losses and the PTOP equation

includes an additional term for transition losses, which are

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

used to estimate the amount of transition loss. The bottom

MOSFET losses are greatest at high input voltage or during

a short circuit when the duty cycle is nearly 100%. The

temperature rise of the MOSFETs depends on the effective

thermal resistance

θJA of the heat sink used in the applica-

tion. Check the temperature of the MOSFET when testing

applications and use appropriate heat sinking such as

board power planes to spread the heat.

Figure 4. SYNC Clock Waveform

Operating Frequency and Synchronization

The choice of operating frequency and inductor value is a

trade-off between efficiency and component size. Low

frequency operation improves efficiency by reducing

MOSFET switching losses, both gate charge loss and

transition loss. However, lower frequency operation

requires more inductance for a given amount of ripple

current.

The internal oscillator runs at a nominal 150kHz frequency

when the SYNC pin is left open or connected to ground.

Pulling the SYNC pin above 1.2V will increase the fre-

quency by 50%. The oscillator will injection lock to a clock

signal applied to the SYNC pin with a frequency between

165kHz and 200kHz. The clock high level must exceed

1.2V for at least 1

µs and no longer than 4µs as shown in

Figure 4. The top MOSFET turn-on will synchronize with

the rising edge of the clock.

1775 F04

µs < tON < 4µs

µs < t < 6µs

1.2V

Inductor Value Selection

Given the desired input and output voltages, the inductor

value and operating frequency directly determine the

ripple current:

∆I

OUT

= 















()( )

–

Lower ripple current reduces losses in the inductor, ESR

losses in the output capacitors and output voltage ripple.

Thus, highest efficiency operation is obtained at low

frequency with small ripple current. To achieve this, how-

ever, requires a large inductor.

APPLICATIO S I FOR ATIO

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