FAN6520A_06 Datasheet, PDF (6/15 Pages) Fairchild Semiconductor – Single Synchronous Buck PWM Controller

Circuit Description
Initialization
The FAN6520A 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 Over-Current Protection (OCP) sample-and-hold
operation. Upon completion of the OCP sample-and-hold
operation, the POR function initiates soft-start operation.
Over-Current Protection
The over-current function protects the converter from a
shorted output by using the upper MOSFET’s on-resis-
tance, R_DS(ON), to monitor the current. This method
enhances the converter’s efficiency and reduces cost by
eliminating the need for a current-sensing resistor. The
over-current function cycles the soft-start function in a
hiccup mode to provide fault protection. A resistor (R_OC-
SET^{) programs the over-current trip level (see Typical}
Application diagram). Immediately following POR, the
FAN6520A initiates the Over-Current Protection sam-
pling-and-hold operation. First, the internal error ampli-
fier is disabled. This allows an internal 20µA current sink
to develop a voltage across R_OCSET. The FAN6520A
then samples this voltage at the COMP pin. This sam-
pled voltage, which is referenced to the VCC pin, is held
internally as the over-current set point. When the voltage
across the upper MOSFET, which is also referenced to
the VCC pin, exceeds the over-current set point, the
over-current function initiates a soft-start sequence. Fig-
ure 3 shows the inductor current after a fault is intro-
duced while running at 15A. The continuous fault causes
the FAN6520A to go into a hiccup mode with a typical
period of 25ms. The inductor current increases to 18A
during the soft-start interval and causes an over-current
trip. The converter dissipates very little power with this
method. The measured input power for the conditions
shown in Figure 3 is 1.5W.
OUTPUT INDUCTOR
CURRENT
5A/DIV.
The over-current function trips at a peak inductor current
(I_PEAK) determined by:
I_PEAK
=
I--_{O----C----S----E---T----}×_{-----R-----O----C---S----E---T--}
R_DS(ON)
(1)
where I_OCSET is the internal OCSET current source
(20µA typical). The OC trip point varies mainly due to the
MOSFET’s R_DS(ON) variations. To avoid over-current
tripping in the normal operating load range, find the R_OC-
SET ^{resistor from the equation above with:}
• The maximum R_DS(ON) at the highest junction
temperature
• The minimum I_OCSET from the specification table
•
Determine
I_PEAK
for
I_PEAK
>
I_OUT(MAX)
+
Δ_-----I
2
where ΔI is the output inductor ripple current.
For an equation for the ripple current, see “Output Induc-
tor (L_out)” under Component Selection.
Internal circuitry of the FAN6520A does not recognize a
voltage drop across R_OCSET larger than 0.5V. Any volt-
age drop across R_OCSET greater than 0.5V sets the over-
current trip point to:
I_PEAK
=
------0---.--5---V--------
R_DS(ON)
(2)
An over-current trip cycles the soft-start function.
Soft-Start
The POR function initiates the soft-start sequence after
the over-current set point has been sampled. Soft-start
clamps the error amplifier output (COMP pin) and refer-
ence input (noninverting terminal of the error amp) to the
internally generated soft-start voltage. Figure 4 shows a
typical start-up interval where the COMP/OCSET pin has
been released from a grounded (system shutdown)
state. Initially, the COMP/OCSET is used to sample the
over-current set point by disabling the error amplifier and
drawing 20µA through R_OCSET. Once the over-current
level has been sampled, the soft-start function is initi-
ated. The clamp on the error amplifier (COMP/OCSET
pin) initially controls the converter’s output voltage during
soft-start. The oscillator’s triangular waveform is com-
pared to the ramping error amplifier voltage. This gener-
ates SW pulses of increasing width that charge the
output capacitor(s). When the internally generated soft-
start voltage exceeds the feedback (FB pin) voltage, the
output voltage is in regulation. This method provides a
rapid and controlled output voltage rise. The entire start-
up sequence typically takes about 11ms.
Figure 3. Over-Current Operation
©2005 Fairchild Semiconductor Corporation
FAN6520A Rev. 1.0.5
6
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