LTC3602EFE datasheet(8/20 Pages) LINER

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LTC3602

3602fb

OPERATION

Dropout Operation

When the input supply voltage decreases toward the output

voltage, the duty cycle increases toward the maximum

on-time. Further reduction of the supply voltage forces the

top switch to remain on for more than one cycle until it

attempts to stay on continuously. In order to replenish the

voltage on the ﬂoating BOOST supply capacitor, however,

the top switch is forced off and the bottom switch is forced

on for approximately 85ns every sixteen clock cycles. This

achieves an effective duty cycle that can exceed 99%. The

output voltage will then be primarily determined by the

input voltage minus the voltage drop across the upper

internal N-channel MOSFET and the inductor.

Slope Compensation and Inductor Peak Current

Slope compensation provides stability in constant-fre-

quency architectures by preventing subharmonic oscilla-

tions at duty cycles greater than 50%. It is accomplished

internally by adding a compensating ramp to the inductor

current signal at duty cycles in excess of 30%. Normally,

the maximum inductor peak current is reduced when

slope compensation is added. In the LTC3602, however,

slope compensation recovery is implemented to reduce

the variation of the maximum inductor peak current (and

therefore the maximum available output current) over the

range of duty cycles.

Short-Circuit Protection

When the output is shorted to ground, the inductor cur-

rent decays very slowly during a single switching cycle.

To prevent current runaway from occurring, a secondary

current limit is imposed on the inductor current. If the

inductor valley current increases to more than 4.5A, the

top power MOSFET will be held off and switching cycles

will be skipped until the inductor current is reduced.

Overtemperature Protection

When using the LTC3602 in an application circuit, care

must be taken not to exceed any of the ratings speci-

ﬁed in the Absolute Maximum Ratings section. As an

added safeguard, however, the LTC3602 does incorporate

an overtemperature shutdown feature. If the junction

temperature reaches approximately 150°C, both power

switches will be turned off and the SW node will become

high impedance. After the part has cooled to below 115°C,

it will restart.

Voltage Tracking and Soft-Start

Some microprocessors and DSP chips need two power

supplies with different voltage levels. These systems often

require voltage sequencing between the core power supply

and the I/O power supply. Without proper sequencing,

latch-up failure or excessive current draw may occur that

could result in damage to the processor’s I/O ports or the

I/O ports of a supporting system device such as memory,

an FPGA or a data converter. To ensure that the I/O loads

are not driven until the core voltage is properly biased,

tracking of the core supply and the I/O supply voltage is

necessary.

Voltage tracking is enabled by applying a ramp voltage to

the TRACK/SS pin. When the voltage on the TRACK pin

is below 0.6V, the feedback voltage will regulate to this

tracking voltage. When the tracking voltage exceeds 0.6V,

tracking is disabled and the feedback voltage will regulate

to the internal reference voltage.

The TRACK/SS pin is also used to implement an external

soft-start function. A 1.2μA current is sourced from this

pin so that an external capacitor may be added to create

a smooth ramp. If this ramp is slower than the internal

1ms soft-start, then the output voltage will track this ramp

during start up instead. Leave this pin ﬂoating to use the

internal 1ms soft-start ramp. Do not tie the TRACK/SS

pin to INTVCC or to PVIN.

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