ISL62381, ISL62382, ISL62383, ISL62381C, ISL62382C, ISL62383C
transition is automatically achieved by detecting the load
current and turning off LGATE when the inductor current
reaches 0A.
Positive-going inductor current flows from either the source
of the high-side MOSFET, or the drain of the low-side
MOSFET. Negative-going inductor current flows into the
drain of the low-side MOSFET. When the low-side MOSFET
conducts positive inductor current, the phase voltage will be
negative with respect to the GND and PGND pins.
Conversely, when the low-side MOSFET conducts negative
inductor current, the phase voltage will be positive with
respect to the GND and PGND pins. These controllers
monitor the phase voltage when the low-side MOSFET is
conducting inductor current to determine its direction.
When the output load current is greater than or equal to ½
the inductor ripple current, the inductor current is always
positive, and the converter is always in CCM. These
controllers minimize the conduction loss in this condition by
forcing the low-side MOSFET to operate as a synchronous
rectifier.
When the output load current is less than ½ the inductor
ripple current, negative inductor current occurs. Sinking
negative inductor current through the low-side MOSFET
lowers efficiency through unnecessary conduction losses.
These controllers automatically enter DEM after the PHASE
pin has detected positive voltage and LGATE was allowed to
go high for eight consecutive PWM switching cycles. These
controllers will turn off the low-side MOSFET once the phase
voltage turns positive, indicating negative inductor current.
These controllers will return to CCM on the following cycle
after the PHASE pin detects negative voltage, indicating that
the body diode of the low-side MOSFET is conducting
positive inductor current.
Efficiency can be further improved with a reduction of
unnecessary switching losses by reducing the PWM
frequency. It is characteristic of the R3 architecture for the
PWM frequency to decrease while in diode emulation. The
extent of the frequency reduction is proportional to the
reduction of load current. Upon entering DEM, the PWM
frequency makes an initial step-reduction because of a 33%
step-increase of the window voltage VW.
Because the switching frequency in DEM is a function of
load current, very light load conditions can produce
frequencies well into the audio band. This can be
problematic if audible noise is coupled into audio amplifier
circuits. To prevent this from occurring, these controllers
allow the user to float the FCCM input. This will allow DEM at
light loads, but will prevent the switching frequency from
going below ~28kHz to prevent noise injection into the audio
band. A timer is reset each PWM pulse. If the timer exceeds
30µs, LGATE is turned on, causing the ramp voltage to
reduce until another UGATE is commanded by the voltage
loop.
Overcurrent Protection
The overcurrent protection (OCP) setpoint is programmed
with resistor, ROCSET, that is connected across the OCSET
and PHASE pins.
PHASE1
ISL62381
DCR
L
IL
+
VDCR
_
ROCSET
CSEN
10µA
OCSET1
+ VROCSET _
RO
ISEN1
VO
CO
FIGURE 26. OVERCURRENT-SET CIRCUIT
Figure 26 shows the overcurrent-set circuit for SMPS1. The
inductor consists of inductance L and the DC resistance
(DCR). The inductor DC current IL creates a voltage drop
across DCR, given by Equation 6:
VDCR = IL DCR
(EQ. 6)
Theses controllers sink a 10µA current into the OCSET1 pin,
creating a DC voltage drop across the resistor ROCSET,
given by Equation 7:
VROCSET = 10A ROCSET
(EQ. 7)
Resistor RO is connected between the ISEN1 pin and the
actual output of the converter. During normal operation, the
ISEN1 pin is a high impedance path, therefore there is no
voltage drop across RO. The DC voltage difference between
the OCSET1 pin and the ISEN1 pin can be established using
Equation 8:
VOCSET1–VISEN1 = IL DCR – 10A ROCSET
(EQ. 8)
These controllers monitor the OCSET1 pin and the ISEN1 pin
voltages. Once the OCSET1 pin voltage is higher than the
ISEN1 pin voltage for more than 10µs, these controllers
declare an OCP fault. The value of ROCSET is then written as
Equation 9:
ROCSET = -I-O-----1C---0----D----A-C-----R---
(EQ. 9)
Where:
- ROCSET () is the resistor used to program the
overcurrent setpoint
- IOC is the output current threshold that will activate the
OCP circuit
- DCR is the inductor DC resistance
For example, if IOC is 20A and DCR is 4.5m, the choice of
ROCSET is ROCSET = 20A x 4.5m/10µA = 9k
16
FN6665.5
May 13, 2011