LT8350RVPBF datasheet(17/24 Pages) AD

17 / 24 page

LT8350

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Frequency Synchronization

The LT8350 switching frequency can be synchronized

to an external clock using the SYNC/MODE pin. Driving

the SYNC/MODE with 10% to 90% duty cycle waveform

is recommended. Due to the use of a phase-locked loop

(PLL) inside, there is no restriction between the synchro-

nization frequency and the internal oscillator frequency.

The rising edge of the synchronization clock represents

the beginning of a switching cycle, turning on switches A

and C, or switches A and D.

Maximum Output Current

The LT8350 is a constant-voltage constant-current buck-

boost converter. The output voltage is regulated up to

the current limit threshold, which is set by the CTRL pin

voltage and the current sense resistor across the ISP/

ISN pins. If the accurate current limit is not required in

the application, connect the CTRL pin to VREF and short

the ISP and ISN pins to disable this accurate current

limit function. Refer to the Programming Input or Output

Current Limit section for more details.

In practice, the maximum output current can be limited by

the thermal constraints of the application. Figure 11 shows

the measured output currents over VIN that increase the

case temperature by 60°C. The output is regulated at 12V,

and the ambient temperature is 25°C. The measurements

were done using the LT8350 demo board. Figure 11 can

be a reference to estimate how much output current and

power the LT8350 can provide under the temperature

constraint for a given application.

LT8350 DEMOBOARD EVAL-LT8350-AZ

VOUT = 12V

TA = 25°C

fSW = 2MHz

fSW = 350kHz

VIN (V)

12 16 20 24 28 32 36 40

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

8350 F11

Figure 11. LT8350 Output Currents Resulting

60°C Case Temperature Increase

Another factor that can limit the output current and power

from the LT8350 is the maximum switch current limit.

The typical maximum switch current limit is 6.8A. When

the output current and power increase for a given input

voltage, the input current will increase until the peak of

the inductor current reaches the maximum switch current

limit. If the load demands more current, the output voltage

may decrease as the converter will regulate the peak of

the inductor current to be less than the maximum switch

current limit.

Inductor Selection

The switching frequency and inductor selection are inter-

related in that higher switching frequencies allow the use

of smaller inductor and capacitor values. The inductor

value has a direct effect on ripple current. The highest cur-

rent ripple ∆IL% happens in the buck region at PVIN(MAX),

and the lowest current ripple ∆IL% happens in the boost

region at VIN(MIN). For any given ripple allowance, the

minimum inductance can be calculated as:

LBUCK >

VOUT • (VIN(MAX) – VOUT)

fSW • IOUT(MAX) • ∆IL% • VIN(MAX)

LBOOST >

VIN(MIN) • (VOUT – VIN(MIN))

fSW • IOUT(MAX) • ∆IL% • VOUT

where:

fSW is switching frequency

∆IL% is allowable inductor current ripple

VIN(MIN) is minimum input voltage

VIN(MAX) is maximum input voltage

VOUT is output voltage

IOUT(MAX) is maximum output current

Slope compensation provides stability in constant fre-

quency current mode control by preventing subharmonic

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