ISL62381 Datasheet, PDF (16/23 Pages) Intersil Corporation – High-Efficiency, Quad or Triple-Output System Power Supply Controller for Notebook Computers

ISL62381, ISL62382, ISL62383
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. The ISL62381,
ISL62382 and ISL62383 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. The ISL62381,
ISL62382 and ISL62383 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.
The ISL62381, ISL62382 and ISL62383 automatically enter
DEM after the PHASE pin has detected positive voltage and
LGATE was allowed to go high for eight consecutive PWM
switching cycles. The ISL62381, ISL62382 and ISL62383
will turn off the low-side MOSFET once the phase voltage
turns positive, indicating negative inductor current. The
ISL62381, ISL62382 and ISL62383 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 V_W.
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, the ISL62381,
ISL62382 and ISL62383 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, R_OCSET, that is connected across the OCSET
and PHASE pins.
PHASE1
ISL62381
DCR
L
I_L
+
V_DCR
_
R_OCSET
C_SEN
10µA
OCSET1
+ ^VROCSET ^_
R_O
ISEN1
V_O
C_O
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 I_L creates a voltage drop
across DCR, given by Equation 6:
V_DCR = I_L • DCR
(EQ. 6)
The ISL62381, ISL62382 and ISL62383 sink a 10µA current
into the OCSET1 pin, creating a DC voltage drop across the
resistor R_OCSET, given by Equation 7:
V_ROCSET = 10μA • R_OCSET
(EQ. 7)
Resistor R_O 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 R_O. The DC voltage difference between
the OCSET1 pin and the ISEN1 pin can be established using
Equation 8:
V_OCSET1–V_ISEN1 = I_L • DCR – 10μA • R_OCSET
(EQ. 8)
The ISL62381, ISL62382 and ISL62383 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, the ISL62381, ISL62382 and ISL62383 declare an
OCP fault. The value of R_OCSET is then written as
Equation 9:
R_OCSET
=
-^I-^O-----^C-----^•--^D-----^C-----^R---
10 μ A
(EQ. 9)
Where:
- R_OCSET (Ω) is the resistor used to program the
overcurrent setpoint
- I_OC is the output current threshold that will activate the
OCP circuit
- DCR is the inductor DC resistance
For example, if I_OC is 20A and DCR is 4.5mΩ, the choice of
R_OCSET is R_OCSET = 20A x 4.5mΩ/10µA = 9kΩ.
16
FN6665.4
August 7, 2008