Data Sheet

LT8350S

analog.com

Rev. A

27 of 36

Maximum Output Current

The LT8350S is a constant-voltage, constant-current, and 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. See 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 38 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 LT8350S

demo board. See Figure 38 to estimate how much output current and power the LT8350S can provide under the

temperature constraint for a given application.

Another factor that can limit the output current and power from the LT8350S 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.

Figure 38. LT8350S Output Currents Resulting 60°C Case Temperature Increase

Inductor Selection

The switching frequency and inductor selection are interrelated in that higher switching frequencies allow the use of

smaller inductor and capacitor values. The inductor value has a direct effect on the ripple current. The highest

current 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)

2

× (VOUT – VIN(MIN))

fSW × IOUT(MAX) × ∆IL% × VOUT

2

where:

fSW is switching frequency

∆IL% is allowable inductor current ripple

VIN(MIN) is minimum input voltage

VIN(MAX) is maximum input voltage

IOUT(MAX) is maximum output current

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

4

8

12

16

20

24

28

32

36

40

fSW = 2MHz

fSW = 350kHz

VIN (V)