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LT8603

8603f

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trip level is 50mV. ISP4 and ISN4 have an operational

common range of 2V to 42V and an absolute maximum

rating of ±42V. This allows reverse battery protection with

a single diode in series with external MOSFET drain.

The logic block turns on the external MOSFET on a CLK4

signal, and then turns it off on an ITRIP from the current

comparator, or when maximum duty cycle is reached.

The LT8603 has a frequency divider that allows the con-

troller switching frequency to be less than the oscilla-

tor frequency. The boost controller is specified for a fSW

between 250kHz and 2.2MHz; do not set the divider to

operate the boost controller below 250kHz. The FSEL4A

and FSEL4B pins control the oscillator frequency as

shown in Table 2.

Table 2. Boost Frequency Selection

BOOST FREQ fSW4

FSEL4A

FSEL4B

0 (Boost Shutdown)

Low

fOSC/5

High

Low

fOSC/2

Low

High

fOSC

High

The boost controller is enabled by the FSEL4A and FSEL4B

pins. When both pins are low, the boost controller is shut

down. When either or both pins are set high, a boost

controller start-up sequence is initiated. During start-up,

the INTVCC4 regulator is turned on charging the external

INTVCC4 capacitor. Once INTVCC4 reaches 4V, the control-

ler logic is turned on and switching begins. A 1ms soft-

start ramp is applied to the error amplifier to minimize

inrush current.

The INTVCC4 regulator is a low dropout, linear regulator

that provides power to the GATE4 drive circuit. It is dedi-

cated to the boost controller and is shut down when the

boost controller is shut down. It can draw power from

either VIN or BIAS, depending on the BIAS voltage. When

BIAS is less that 4.6V, the regulator will draw power from

VIN and regulate to 4.6V. When BIAS is greater than 4.6V

but less than 5V, the regulator sets INTVCC4 equal to BIAS

and draws power from BIAS. When BIAS is greater than

5V, the regulator generates 5V and draws power from

BIAS. Do not use the INTVCC4 output for external circuitry.

A comparator senses when the output voltage is above

92% of the programmed value and generates a power

good signal at the PG4 pin.

Setting the Output Voltage

The boost controller’s output voltage is set by connecting

the FB4 pin to a resistor divider from the output as shown

in Figure 3.

APPLICATIONS INFORMATION

Figure 3. Feedback Resistor Divider

The value of R2 is best selected first as this establishes

how much current is in the string based on I = VFB4/R2

where VFB4 = 0.8V. The current should be chosen such

that it is not influenced by anticipated leakage or noise.

R1 can then be calculated from:

R1 = R2 • ((VOUT4/VFB4) – 1)

Boost Configuration

The LT8603 boost controller configured as a standard

boost regulator is shown in Figure 4. The boost control-

ler output voltage is set by connecting FB4 to an external

divider. For the following example, R2 and R3 are chosen

such that OUT4 = 8V. If the battery voltage (VBATT) is

above 8V plus a diode forward voltage, the boost control-

ler will be inactive consuming minimal quiescent current.

If the battery voltage drops below 8V plus a diode forward

voltage, such as during an automotive cold crank cycle,

the boost controller will become active to maintain 8V

at OUT4. With OUT4 connected to VIN, the input supply

must reach 4V plus the diode forward voltage to start.

8603 F03

FB4

VOUT4

LT8603

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