LCS700-708 datasheet(14/26 Pages) POWERINT

14 / 26 page

Rev. B 062011

LCS700-708

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For example, if BT2 is selected, and f

MAX is 800 kHz, then fSTART

= 300 kHz, and f

STOP = 350 kHz. If during normal operation the

load is reduced and the frequency rises to 350 kHz, the switching

will stop. This causes the output voltage to drop and the

feedback loop to decrease the FEEDBACK pin current. When

the current decreases to a value which corresponds to 300 kHz,

switching will commence, and the cycle will repeat. During

start-up mode, however, the outputs can switch at a frequency

between f

STOP and fMAX (250 kHz and 800 kHz in the above

example). Start-up mode is exited once the switching frequency

drops below f

STOP, and the HiperLCS will subsequently enter

burst mode if the feedback loop attempts to produce a

switching frequency >f

STOP.

MAX is the frequency at which the internal counters run when the

HiperLCS is in the off-state of the auto-restart cycle, or in the

power-up delay before switching.

The minimum recommended dead-time is 275 ns, and thus the

maximum f

MAX setting is 1 MHz.

To simplify the selection of R

FMAX, see the selection curves in Figure 17.

Figure 16. FEEDBACK Pin and DT/BF Pin Current vs. Frequency.

600

400

200

800

1000

150

100

200

250

300

350

400

450

Frequency (kHz)

250

300

350

400

500

450

Dead-Time (ns)

13.0

12.0

11.0

9.0

10.0

8.0

7.0

6.0

5.0

BT1

BT2

BT3

Figure 17. R

FMAX vs. Dead-Time, for the 3 Different Burst Threshold Settings.

Figure 18. f

START (Lower Burst Threshold Frequency) vs. Dead-Time Setting for

Different Burst Threshold Settings (BT1, BT2, BT3).

250

300

350

400

500

450

Dead-Time (ns)

500

450

350

400

300

250

200

150

BT1

BT2

BT3

The f

STOP to fSTART ratio is fixed, and dependent on the Burst

Threshold setting (see Table 5).

Table 5. Ratio of f

STOP /fSTART vs. Burst Threshold Selection.

As a first approximation, during burst mode, the frequency

ramps from f

START to fSTOP; then switching stops, and then the cycle

repeats.

FEEDBACK Pin

The FEEDBACK pin is the voltage regulation FEEDBACK pin. It

has a nominal Thevenin equivalent circuit of 0.65 V and 2.5 kW.

In normal operation, it sinks current. During the off-period of

auto-restart, and during the clocked delay before start-up, it pulls

up internally to V

REF in order to discharge the soft-start capacitor.

The current entering the pin determines switching frequency.

Higher current yields higher frequency and thus reduces LLC

output voltage. In a typical application an optocoupler connected

to the VREF pin pulls up on the FEEDBACK pin, via a resistor

network. The optocoupler is configured to source increasing

FEEDBACK pin current, as the output rises. The resistor network

between the optocoupler, FEEDBACK pin, and VREF pin,

determine the minimum and maximum FEEDBACK pin current

(and thus the minimum and maximum operating frequency), that

the optocoupler can command as it goes from cutoff to saturation.

This network also contains the soft-start timing capacitor, C

START

(Figure 19).

The minimum frequency as set by this network must be lower

than the frequency required by the powertrain at minimum input

voltage. In Figure 19 this is determined by the sum of R

FMIN and

START. The FEEDBACK pin current is determined by these two

resistors when the optocoupler is cut off. C

START can be ignored

during normal operation. Do not confuse R

START, which determines

Burst Threshold

Setting

STOP / fSTART

1.14

1.17

1.20

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