FAN6520B Fairchild Semiconductor, FAN6520B Datasheet - Page 6

• Manufacturer Part Number: FAN6520B
• Description: Single Synchronous Buck PWM Controller
• Manufacturer: Fairchild Semiconductor

FAN6520B Rev. 1.0.3
Circuit Description
Initialization
The FAN6520B automatically initializes upon receipt of
power. The Power-On Reset (POR) function continually
monitors the bias voltage at the VCC pin. When the sup-
ply voltage exceeds its POR threshold, the IC initiates
the soft-start operation.
Soft-Start
The POR function initiates the soft-start sequence. Soft-
start clamps the error amplifier output (COMP pin) and
reference input (noninverting terminal of the error amp)
to the internally generated soft-start voltage. Figure 3
shows a typical start up interval where the COMP pin
has been released from a grounded (system shutdown)
state. The clamp on the error amplifier (COMP pin) ini-
tially controls the converter’s output voltage during soft-
start. The oscillator’s triangular waveform is compared to
the ramping error amplifier voltage. This generates SW
pulses of increasing width that charge the output capaci-
tor(s). When the internally generated soft-start voltage
exceeds the feedback (FB pin) voltage, the output volt-
age is in regulation. This method provides a rapid and
controlled output voltage rise. The entire startup
sequence typically takes about 11ms.
Adaptive Gate Drive
The FAN6520B incorporates a MOSFET shoot-through
protection method which allows a converter to both sink
and source current. Care should be exercised when
designing a converter with the FAN6520B when it is
known that the converter may sink current.
When the converter is sinking current, it is behaving as a
boost converter that is regulating its input voltage. This
means that the converter is boosting current into the
VCC rail, which supplies the bias voltage to the
Figure 3. Soft-Start Interval
6
FAN6520B. If this current has nowhere to go—such as to
other distributed loads on the VCC rail, through a voltage
limiting protection device, or other methods—the capaci-
tance on the VCC bus will absorb the current. This situa-
tion will allow the voltage level of the VCC rail to
increase. If the voltage level of the rail is boosted to a
level that exceeds the maximum voltage rating of the
FAN6520B, then the IC will experience an irreversible
failure and the converter will no longer be operational.
Ensure that there is a path for the current to follow other
than the capacitance on the rail to prevent this failure
mode.
Application Guidelines
Layout Considerations
As in any high frequency switching converter, layout is
very important. Switching current from one power device
to another can generate voltage transients across the
impedances of the interconnecting bond wires and circuit
traces. Use wide, short-printed circuit traces to minimize
these interconnecting impedances. The critical com-
ponents should be located as close together as possible,
using ground plane construction or single point
grounding.
Figure 4 shows the critical power components of the
converter. To minimize the voltage overshoot, the
interconnecting wires indicated by heavy lines should be
part of a ground or power plane in a printed circuit board.
The components shown in Figure 4 should be located as
close together as possible. Please note that the capa-
citors C
physical capacitors. Locate the FAN6520B as close as
possible to Q1 and Q2 MOSFETs. The circuit traces for
the MOSFETs’ gate and source connections from the
FAN6520B must be sized to handle up to 1A peak
current.
FAN6520B
Figure 4. Printed Circuit Board Power and
IN
HDRV
LDRV
SW
and C
Ground Planes or Islands
OUT
may each represent numerous
Q1
Vin
Q2
L
OUT
C
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IN
C
+V
OUT
OUT