LTC3802 Datasheet, PDF(17/28 Page) Linear Technology – Dual 550kHz Synchronous 2-Phase DC/DC Controller with Programmable Up/Down Tracking

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LTC3802 Datasheet, PDF (17/28 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase DC/DC Controller with Programmable Up/Down Tracking

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LTC3802

APPLICATIO S I FOR ATIO

special power supplies with a slow start-up slew rate, the

LTC3802 drivers might start switching before the input

supply reaches its steady-state value. The high inrush

current through the input power cable might cause the VIN

supply to dip below the UVLO threshold and cause start-

up problems. Figureâ£ 3 shows a simple circuit to fix this

problem. The selection of the zener voltage allows the VIN

UVLO trip point to be programmed externally.

1N4699

VZ = 12V

100k

10k

VIN > VZ

100k

2N3904

LTC3802

RUN/SS

CSS

3802 F03

Figure 3. External UVLO Setting

Start-Up Tracking

Many DSP chips, microprocessors, FPGAs and ASICs

require multiple power supplies for the core and I/O

sections. Internally, the core and I/O blocks are isolated by

structures which may become forward biased if the supply

voltages are not at specified levels. During power-up and

power-down operations, differences in the starting point

and ramp rates of the two supplies may cause current to

flow between the isolation structures which, when pro-

longed and excessive, can reduce the useable life of the

semiconductor device. These currents can also trigger

latch-up in devices, leading to device failure.

Of greater concern than internal isolation of core and I/O

structures are system-level concerns, such as bus

contention between the I/O pins of the DSP and external

peripheral devices. Power supply sequencing between the

core and I/O may be required to prevent bidirectional I/O

pins of the DSP and a peripheral device from opposing

each other. Since the bus control logic originates in the

core section, powering the I/O prior to the core may cause

the DSP and peripheral pins to be configured

simulatneously as outputs. If the data values on each side

are opposing, then the output drivers contend for control,

causing excessive current flow and eventually device

failure.

The LTC3802 can be configured to give two different

power-up/power-down slew rates to meet different appli-

cation requirements: ratiometric and coincident tracking

configurations (Figure 4). With a ratiometric configura-

tion, the LTC3802 produces two different output slew

rates (with VOUT1 > VOUT2). Because each channelâs slew

rate is proportional to its corresponding output voltage,

the two output voltages reach their steady-state value at

about the same time. The coincident configuration pro-

duces the same slew rate at both outputs, so that the lower

output voltage channel reaches its steady state first.

Figure 4 shows the simplified schematic to realize this

power-up function. During power-up, the tracking ampli-

fier TRACK servos the tracking feedback loop and forces

FBT to be at the same potential as CMPIN2.

For ratiometric start-up, set:

RT5 = R51

or remove resistors RT4 and RT5 and short FBT to CMPIN1.

At power-up, if the channel 2 output voltage slew rate is

too fast, or CMPIN2 is higher than FBT, the tracking

amplifier will force a smaller channel 2 duty cycle.

Channelâ£ 1âs duty cycle is controlled by the RUN/SS pin and

is not affected by the tracking amplifier.

For coincident start-up, set:

RT5 = R52

During power-up, if the channel 1 output voltage is higher

than that of channel 2, or if FBT is higher than CMPIN2, the

tracking amplifier TRACK starts to discharge the CSS

capacitor and forces both channels to have the same duty

cycle and output voltage. The tracking amplifier stops

discharging once channel 2 reaches its negative power

good threshold.

To have the proper power-down sequence, ground the

PHASEMD pin. This turns on an internal current source

which slowly discharges the soft-start capacitor. Once the

RUN/SS potential is low enough to control the duty cycle,

the tracking amplifier takes control and servos the feed-

back loop to produce the selected output ramp. The

LTC3802 tracking function can be easily disabled by

disconnecting the FBT resistive divider and shorting FBT

to CMPIN2.

3802f

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