ISL62873 Datasheet, PDF (11/17 Pages) Intersil Corporation – PWM DC/DC Controller with VID Inputs for Portable GPU Core-Voltage Regulator

ISL62873
converter. The fault will remain latched until the EN pin has
been pulled below the falling EN threshold voltage V_ENTHF
or if VCC has decayed below the falling POR threshold
voltage ^V_{VCC_THF}.
Over-Temperature
When the temperature of the IC increases above the rising
threshold temperature T_OTRTH, it will enter the OTP state
that suspends the PWM, forcing the LGATE and UGATE
gate-driver outputs low. The status of the PGOOD pin does
not change nor does the converter latch-off. The PWM
remains suspended until the IC temperature falls below the
hysteresis temperature T_OTHYS at which time normal PWM
operation resumes. The OTP state can be reset if the EN pin
is pulled below the falling EN threshold voltage V_ENTHF or if
VCC has decayed below the falling POR threshold voltage
V_{VCC_THF. All other protection circuits remain functional}
while the IC is in the OTP state. It is likely that the IC will
detect an UVP fault because in the absence of PWM, the
output voltage decays below the undervoltage threshold
V_UVTH.
Theory of Operation
The modulator features Intersil’s R³ Robust-Ripple-
Regulator technology, a hybrid of fixed frequency PWM
control and variable frequency hysteretic control. The PWM
frequency is maintained at 300kHz under static
continuous-conduction-mode operation within the entire
specified envelope of input voltage, output voltage, and
output load. If the application should experience a rising load
transient and/or a falling line transient such that the output
voltage starts to fall, the modulator will extend the on-time
and/or reduce the off-time of the PWM pulse in progress.
Conversely, if the application should experience a falling
load transient and/or a rising line transient such that the
output voltage starts to rise, the modulator will truncate the
on-time and/or extend the off-time of the PWM pulse in
progress. The period and duty cycle of the ensuing PWM
pulses are optimized by the R³ modulator for the remainder
of the transient and work in concert with the error amplifier
V_ERR to maintain output voltage regulation. Once the
transient has dissipated and the control loop has recovered,
the PWM frequency returns to the nominal static 300kHz.
Modulator
The R³ modulator synthesizes an AC signal V_R, which is an
analog representation of the output inductor ripple current.
The duty-cycle of V_R is the result of charge and discharge
current through a ripple capacitor C_R. The current through
C_R is provided by a transconductance amplifier g_m that
measures the input voltage (V_IN) at the PHASE pin and
output voltage (V_OUT) at the VO pin. The positive slope of
V_R can be written as Equation 19:
V_RPOS = (g_m) ⋅ (V_IN – V_OUT) ⁄ C_R
(EQ. 19)
11
The negative slope of V_R can be written as Equation 20:
V_RNEG = g_m ⋅ V_OUT ⁄ C_R
(EQ. 20)
Where, g_m is the gain of the transconductance amplifier.
A window voltage V_W is referenced with respect to the error
amplifier output voltage V_COMP, creating an envelope into
which the ripple voltage V_R is compared. The amplitude of
V_W is controlled internally by the IC. The V_R, V_COMP, and
V_W signals feed into a window comparator in which V_COMP
is the lower threshold voltage and V_W is the higher threshold
voltage. Figure 7 shows PWM pulses being generated as V_R
traverses the V_W and V_COMP thresholds. The PWM
switching frequency is proportional to the slew rates of the
positive and negative slopes of V_R; it is inversely
proportional to the voltage between V_W and V_COMP.
RIPPLE CAPACITOR VOLTAGE C_R
WINDOW VOLTAGE V_W
ERROR AMPLIFIER VOLTAGE V_COMP
PWM
FIGURE 7. MODULATOR WAVEFORMS DURING LOAD
TRANSIENT
Synchronous Rectification
A standard DC/DC buck regulator uses a free-wheeling
diode to maintain uninterrupted current conduction through
the output inductor when the high-side MOSFET switches off
for the balance of the PWM switching cycle. Low conversion
efficiency as a result of the conduction loss of the diode
makes this an unattractive option for all but the lowest
current applications. Efficiency is dramatically improved
when the free-wheeling diode is replaced with a MOSFET
that is turned on whenever the high-side MOSFET is turned
off. This modification to the standard DC/DC buck regulator
is referred to as synchronous rectification, the topology
implemented by the ISL62873 controller.
Diode Emulation
The polarity of the output inductor current is defined as
positive when conducting away from the phase node, and
defined as negative when conducting towards the phase
node. The DC component of the inductor current is positive,
but the AC component known as the ripple current, can be
either positive or negative. Should the sum of the AC and
DC components of the inductor current remain positive for
the entire switching period, the converter is in
FN6930.0
June 30, 2009