LCS700-708 datasheet(6/26 Pages) POWERINT

6 / 26 page

Rev. B 062011

LCS700-708

www.powerint.com

pulse width requirement for detection of both voltage thresholds

is nominally 30 ns. i.e. the thresholds have to be exceeded for

>30 ns for proper detection.

Over-Temperature Shutdown

The HiperLCS has latching OTP. VCCH must be cycled to

resume operation once the unit drops down below the OTP

threshold.

Basic Layout Guidelines

The HiperLCS is a high-frequency power device and requires

careful attention to circuit board layout in order to achieve

maximum performance.

The bypass capacitors need to be positioned and laid out

carefully to minimize trace lengths to the pins they serve. SMD

components are recommended for minimum component and

trace stray inductance.

Table 2 describes the recommended bypass capacitor values

for pins that require filtering/bypassing. The table lists the pins

in the order of most to least sensitive. The bypass capacitor of

the pin at the top of the list being the most sensitive, receives

higher priority in bypass capacitor positioning to minimize trace

lengths, than the bypass capacitor of the pin below it. Noise

entering the two most sensitive pins on the list, namely the

FEEDBACK and DT/BF pins, will cause duty cycle, and dead-

time imbalance, respectively.

Figure 5 and Figure 6 show two alternate schemes for routing

ground traces for optimum performance. Figure 5 shows a

layout footprint for the LCS with oval pads. These allow a trace

to be passed between pins 3 and 5, directly connecting the

ground systems for the bypass capacitors located on each side

of the IC.

Figure 6 shows an LCS layout footprint with round pads that do

not allow traces to be routed between them due to insufficient

space. In this case, a jumper (JP1, a 1206 size 0 W resistor) is

used to connect the ground systems together and allow a

connection for pin 3 to be routed under JP1 to the optocoupler.

Transformer T1 is a source of both high di/dt signals and dv/dt

noise. The first can couple magnetically to sensitive circuitry,

while the second can inject noise via electrostatic coupling.

Electrostatic noise coupling can be reduced by grounding the

transformer core, but it is not economically feasible to reduce

the stray magnetic field around the transformer without drastically

reducing its efficiency. Sensitive traces and components (such

as the optocoupler) should be located away from the transformer

to avoid noise pickup.

Returned to Pin

Recommended Value

Notes

FEEDBACK (FB)

GROUND

4.7 nF (at 250 kHz)

Increase value proportionally for lower nominal frequency (e.g. 10

nF at 100 kHz). Forms a pole with FEEDBACK pin input

impedance which is part of feedback loop characteristic. Must

not introduce excessive phase shift at expected gain

crossover frequency. Noise entering FEEDBACK pin will cause

duty cycle imbalance.

DEAD-TIME/BURST

FREQUENCY (DT/BF)

GROUND

4.7 nF

Time constant of this capacitor and the source

impedance of the resistors connected to DT/BF pin

must be <100 ms. Noise entering DT/BF pin will cause

dead time imbalance.

CURRENT SENSE (IS)

GROUND

1 nF (at 250 kHz)

Value changes proportionally with nominal LLC stage

operating frequency. Forms an RC low pass filter with

recommended 220 W series resistor. Must not attenuate

AC signal of primary current sense.

VCC

GROUND

1 mF ceramic

VREF

GROUND

1 mF ceramic

VCCH

0.1 mF - 0.47 mF

Bootstrap capacitor. Provides instantaneous current

for high-side driver for turning on high-side MOSFET.

Time constant formed with boost-strap current limiting

resistor (in series with bootstrap diode), delays VCCH

UVLO for a few switching cycles at start-up and during

burst mode operation for the first switching cycles

DRAIN

(DC Bus)

S1, S2

10-22 nF SMD ceramic

minimum, plus 22-100 nF

through-hole

Total of 22 nF per amp of nominal primary RMS current.

SMD part must be located directly at the IC and

connected close, with short traces. This prevents

ringing of D-S during hard-switching (loss of ZVS)

transients. It also reduces high-frequency EMI.

OV/UV

GROUND

4.7 nF

Table 2.

Bypass Capacitor Table in Order of Importance.

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