LT8602 datasheet(20/38 Pages) LINER

20 / 38 page

LT8603

8603f

For more information www.linear.com/LT8603

Once started however, the LT8603 will maintain the output

voltage at OUT4 with VBATT as low as 2V. The ISP4 and

ISN4 pins sense the inductor current for the current mode

control loop across current sense resistor R1. The ISP4

and ISN4 pins will accurately sense the inductor current

down to 2V and will tolerate negative voltages as large as

–42V without damage.

By adding diode SD2 in series with the main switch tran-

sistor M1, the entire regulator will be tolerant of negative

input supply voltages.

One of the limitations of the boost configuration is that

it cannot provide short-circuit protection for the boost

output as the diode always provides a DC current path

from input to output. If short-circuit overload protection

is required the SEPIC configuration should be considered.

See the SEPIC Configuration section for more information

on the SEPIC converter.

Boost: Duty Cycle and Max Switching Frequency

In continuous conduction mode (CCM), the operating

duty cycle as a function of input and output voltage is

given by:

D =

VOUT + VD – VBATT

VOUT + VD

















where VD is the forward voltage drop of the diode.

APPLICATIONS INFORMATION

Thus the maximum duty cycle (DMAX) in terms of the

minimum VBATT is:

DMAX =

VOUT + VD – VBATT MIN

(

)

VOUT + VD

For a given input and output voltage, the switching fre-

quency is limited by the expected maximum duty cycle,

DMAX, and minimum switch off time, tOFF(MIN). The

fSW(MAX) at DMAX is given by:

fSW(MAX) =

DMAX

tOFF(MIN)

Boost: Inductor and Sense Resistor Selection

The boost regulator inductor and sense resistor (RSENSE)

should be sized according to the maximum input cur-

rent. The maximum input current will occur at the mini-

mum input voltage (maximum duty cycle) and maximum

output load.

The maximum average inductor current, which is equal to

the average input current, can be calculated from:

IL(MAX) =

IOUT(MAX)

1 – DMAX

Figure 4. Channel 4 in Boost Configuration

8603 F04

SD1

VBATT

LT8603

FB4

GATE4

EN/UVLO

INTVCC4

ISN4

ISP4

FSEL4A FSEL4B GND

VIN

SD2*

*SD2 IS FOR REVERSE INPUT PROTECTION. SHORT IF NOT NEEDED.

INTVCC4

INTVCC

VOUT4 =0.8•

(R2+R3)

when VBATT < VOUT4 +VF(SD1)

= VBATT – VF(SD1) = when VBATT ≥ VOUT4 +VF(SD1)

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