ISL6439 Datasheet, PDF (8/15 Pages) Intersil Corporation – Single Sync Buck PWM Controller for Broadband Gateway Applications

ISL6439, ISL6439A
+3.3V
VCC
VIN
CPVOUT
BOOT
ISL6439
UGATE
PHASE
LGATE
D1
C4
Q1
L_OUT
Q2
+
C_OUT
FB
COMP
C1
R1
C3
C2 R2
R4
V_OUT
R3
FIGURE 2. OUTPUT VOLTAGE SELECTION
The overcurrent function cycles the soft-start function in a
hiccup mode to provide fault protection. A resistor (R_OCSET)
programs the overcurrent trip level (see Typical Application
diagrams on pages 2 and 3). An internal 20μA (typical) current
sink develops a voltage across R_OCSET that is referenced to
V_IN. When the voltage across the upper MOSFET (also
referenced to V_IN) exceeds the voltage across R_OCSET, the
overcurrent function initiates a soft-start sequence.
Figure 3 illustrates the protection feature responding to an
overcurrent event. At time t0, an overcurrent condition is
sensed across the upper MOSFET. As a result, the regulator
is quickly shutdown and the internal soft-start function begins
producing soft-start ramps. The delay interval seen by the
output is equivalent to three soft-start cycles. The fourth
internal soft-start cycle initiates a normal soft-start ramp of
the output, at time t1. The output is brought back into
regulation by time t2, as long as the overcurrent event has
cleared.
Had the cause of the over current still been present after the
delay interval, the over current condition would be sensed
and the regulator would be shut down again for another
delay interval of three soft-start cycles. The resulting hiccup
mode style of protection would continue to repeat
indefinitely.
The overcurrent function will trip at a peak inductor current
(I_PEAK) determined by Equation 3:
I_PEAK
=
I_{--O-----C----S----E----T-----}x_-----R_{-----O----C-----S----E---T--}
r_DS(ON)
(EQ. 3)
where I_OCSET is the internal OCSET current source (20μA
typical). The OC trip point varies mainly due to the MOSFET
V_OUT (2.5V)
0V
Internal Soft-Start Function
Delay Interval
T0
T1
T2
TIME
FIGURE 3. OVER CURRENT PROTECTION RESPONSE
r_DS(ON) variations. To avoid overcurrent tripping in the
normal operating load range, find the R_OCSET resistor from
the equation above with:
1. The maximum r_DS(ON) at the highest junction
temperature.
2. The minimum I_OCSET from the specification table.
3. 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 the section under
component guidelines titled ‘Output Inductor Selection’.
A small ceramic capacitor should be placed in parallel with
R_OCSET to smooth the voltage across R_OCSET in the
presence of switching noise on the input voltage.
Current Sinking
The ISL6439 incorporates a MOSFET shoot-through
protection method which allows a converter to sink current
as well as source current. Care should be exercised when
designing a converter with the ISL6439 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 input
rail of the regulator. If there is nowhere for this current to go,
such as to other distributed loads on the rail or through a
voltage limiting protection device, the capacitance on this rail
will absorb the current. This situation will allow the voltage
level of the input rail to increase. If the voltage level of the rail
is boosted to a level that exceeds the maximum voltage
rating of any components attached to the input rail, then
those components may experience an irreversible failure or
experience stress that may shorten their lifespan. Ensuring
8
FN9057.5
November 5, 2008