ISL62391 Datasheet, PDF (14/20 Pages) Intersil Corporation – High-Efficiency, Triple-Output System Power Supply Controller for Notebook Computers

ISL62391, ISL62392
ISL62392 monitors 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 ISL62391,
ISL62392 minimizes 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 through the low-side MOSFET lowers
efficiency through unnecessary conduction losses. The
ISL62391, ISL62392 automatically enters DEM after the
PHASE pin has detected positive voltage and LGATE was
allowed to go high for 8 consecutive PWM switching cycles.
The ISL62391, ISL62392 will turn off the low-side MOSFET
once the phase voltage turns positive, indicating negative
inductor current. The ISL62391, ISL62392 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 ISL62391,
ISL62392 allows 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 to 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.
L
PHASE1
ISL62391,
ISL62392
DCR
I_L
+
V_DCR
_
R_OCSET
C_SEN
10µ
OCSET1
+ ^VROCSET ^_
R_O
OUT1
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, which is given by Equation 6:
V_DCR = I_L • DCR
(EQ. 6)
The ISL62391, ISL62392 sinks a 10µA current into the
OCSET1 pin, creating a DC voltage drop across the resistor
R_OCSET, which is given by Equation 7:
V_ROCSET = 10μA • R_OCSET
(EQ. 7)
Resistor R_O is connected between the OUT1 pin and the
actual output voltage of the converter. During normal
operation, the OUT1 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 OUT1 pin can
be established using Equation 8:
V_OCSET1–V_OUT1 = I_L • DCR – 10μA • R_OCSET
(EQ. 8)
The ISL62391, ISL62392 monitors the OCSET1 pin and the
OUT1 pin voltages. Once the OCSET1 pin voltage is higher
than the OUT1 pin voltage for more than 10µs, the ISL62391,
ISL62392 declares 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.
Resistor R_OCSET and capacitor C_SEN form an R-C network
to sense the inductor current. To sense the inductor current
correctly, not only in DC operation but also during dynamic
operation, the R-C network time constant R_OCSET-C_SEN
14
FN6666.4
December 22, 2008