ISL6228_14 Datasheet, PDF (9/16 Pages) Intersil Corporation – High-Performance Dual-Output Buck Controller for Notebook Applications

ISL6228
pins. The power for the LGATE gate-driver is sourced
directly from the PVCC pin. The power for the UGATE gate-
driver is sourced from a “boot” capacitor connected across
the BOOT and PHASE pins. The boot capacitor is charged
from a 5V bias supply through a “boot diode” each time the
low-side MOSFET turns on, pulling the PHASE pin low. The
ISL6228 has an integrated boot diode connected from the
PVCC pin to the BOOT pin.
t_LGFUGR
UGATE
LGATE
50%
50%
t_UGFLGR
FIGURE 4. LGATE AND UGATE DEAD-TIME
Diode Emulation
The ISL6228 implements forced continuous-conduction-
mode (CCM) at heavy load and diode-emulation-mode
(DEM) at light load, to optimize efficiency in the entire load
range. The transition is automatically achieved by detecting
the output load current.
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. The ISL6228 monitors
the phase voltage, when the low-side MOSFET is
conducting inductor current, to determine the direction of the
inductor current.
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 ISL6228
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
ISL6228 automatically enters DEM after the PHASE pin has
detected positive voltage and LGATE was allowed to go high
for eight consecutive PWM switching cycles. The ISL6228
will turn off the low-side MOSFET once the phase voltage
turns positive, indicating negative inductor current. The
ISL6228 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.
Overcurrent Protection
The overcurrent protection (OCP) setpoint is programmed
with resistor R_OCSET that is connected across the OCSET
and PHASE pins.
PHASE
ISL6228
DCR
L
I_L
+
V_DCR
_
R_OCSET
C_SEN
10µA
OCSET
+ ^VROCSET ^_
R_O
VO
V_O
C_O
FIGURE 5. OVERCURRENT-SET CIRCUIT
Figure 5 shows the overcurrent-set circuit. 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 3:
V_DCR = I_L • DCR
(EQ. 3)
The ISL6228 sinks 10µA current into the OCSET pin,
creating a DC voltage drop across the resistor R_OCSET,
given by Equation 4:
V_ROCSET = 10μA • R_OCSET
(EQ. 4)
Resistor R_O is connected between the VO pin and the actual
output voltage of the converter. During normal operation, the
VO pin is a high impedance path, therefore there is no
voltage drop across R_O. The DC voltage difference between
the OCSET pin and the VO pin can be established using
Equation 5:
V_OCSET–V_VO = V_DCR–V_ROCSET = I_L • DCR – 10μA • R_OCSET
(EQ. 5)
9
FN9095.2
May 7, 2008