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LT8603

8603f

For more information www.linear.com/LT8603

APPLICATIONS INFORMATION

SYSTEM ARCHITECTURE

The LT8603 combines three buck converters with a boost

controller to provide a flexible system supply that can

be configured to generate up to four regulated outputs.

The 4 channels are independently powered and can be

connected in a variety of ways. For example, the output

of the boost may be used to supply the input voltage to

the buck converters resulting in three tightly regulated

outputs even when the boost input voltage falls below

the regulated buck outputs such as occurs during an

automotive cold crank scenario. Alternatively, if the boost

controller is driven from a buck output or is configured as

a SEPIC converter, the LT8603 provides up to four tightly

regulated outputs.

VIN Voltage Range

The minimum voltage at VIN for the LT8603 internal cir-

cuitry and the buck converters to start is 3.1V, however,

at least 4V is required for the boost controller and the

INTVCC4 regulator to start. The boost controller can be

configured to supply VIN and the PVIN pins once it has

started; after start-up the input voltage to the boost con-

troller can go lower than 3V.

Enable and Undervoltage Lockout

The EN/UVLO pin can be used to to program a minimum

system start voltage or an undervoltage lockout (UVLO)

voltage. It has an internal threshold of 1.2V with 50mV

hysteresis. The UVLO divider circuit is shown in Figure 1.

The UVLO threshold is given by:

V UVLO

(

) =

RUV1+RUV2

RUV2

• 1.2V

Switching Frequency

All 4 channels share a single oscillator. The buck chan-

nels switch at the oscillator frequency. The boost chan-

nel can switch at fOSC, fOSC/2 or fOSC/5. The switching

frequency range of all 4 channels should be determined

before selecting the oscillator frequency. A low frequency

usually provides better efficiency and a wider operating

range due to lower switching losses and less sensitivity

to timing constraints such as minimum on- and off-times.

A high switching frequency uses smaller components and

moves the switching noise away from sensitive frequency

bands, such as the AM radio band, but does so at the

cost of lower efficiency. A high switching frequency also

decreases the duty cycle range because of finite mini-

mum on- and off-times which are independent of the

switching frequency.

The oscillator frequency can be programmed from 250kHz

to 2.2MHz by tying a resistor from the RT pin to ground.

Table 1 shows the necessary value of RT for some com-

mon switching frequencies.

Table 1. Oscillator Frequency (fOSC) vs RT Value

OSCILLATOR

FREQUENCY (MHz)

RT (kΩ)

0.25

244

0.35

173

0.5

120

0.75

79.2

1.0

58.9

1.25

46.8

1.5

38.7

1.75

33.0

2.0

28.7

2.2

26.0

The following equation approximates the values shown

in Table 1:

RT =

59.8

(fOSC – 0.007)

– 1.3

The RT pin is sensitive to noise so the resistor should be

placed close to the LT8603 and away from noise sources.

Figure 1. UVLO Divider

8603 F01

EN/UVLO

LT8603

VIN

VIN OR VBATT

RUV1

RUV2

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