ISL62881 Datasheet, PDF (17/35 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62881, ISL62881B
Adaptive Body Diode Conduction Time
Reduction
In DCM, the controller turns off the low-side MOSFET
when the inductor current approaches zero. During
on-time of the low-side MOSFET, phase voltage is
negative and the amount is the MOSFET R_DS(ON) voltage
drop, which is proportional to the inductor current. A
phase comparator inside the controller monitors the
phase voltage during on-time of the low-side MOSFET
and compares it with a threshold to determine the
zero-crossing point of the inductor current. If the
inductor current has not reached zero when the low-side
MOSFET turns off, it’ll flow through the low-side MOSFET
body diode, causing the phase node to have a larger
voltage drop until it decays to zero. If the inductor
current has crossed zero and reversed the direction when
the low-side MOSFET turns off, it’ll flow through the
high-side MOSFET body diode, causing the phase node to
have a spike until it decays to zero. The controller
continues monitoring the phase voltage after turning off
the low-side MOSFET and adjusts the phase comparator
threshold voltage accordingly in iterative steps such that
the low-side MOSFET body diode conducts for
approximately 40ns to minimize the body diode-related
loss.
Overshoot Reduction Function
The ISL62881 has an optional overshoot reduction
function, enabled or disabled by the resistor from the
COMP pin to GND, as shown in Table 3.
When a load release occurs, the energy stored in the
inductors will dump to the output capacitor, causing
output voltage overshoot. The inductor current
freewheels through the low-side MOSFET during this
period of time. The overshoot reduction function turns off
the low-side MOSFET during the output voltage
overshoot, forcing the inductor current to freewheel
through the low-side MOSFET body diode. Since the body
diode voltage drop is much higher than MOSFET R_DS(ON)
voltage drop, more energy is dissipated on the low-side
MOSFET therefore the output voltage overshoot is lower.
If the overshoot reduction function is enabled, the
ISL62881 monitors the COMP pin voltage to determine
the output voltage overshoot condition. The COMP
voltage will fall and hit the clamp voltage when the
output voltage overshoots. The ISL62881 will turn off
LGATE when COMP is being clamped. The low-side
MOSFET in the power stage will be turned off. When the
output voltage has reached its peak and starts to come
down, the COMP voltage starts to rise and is no longer
clamped. The ISL62881 will resume normal PWM
operation.
While the overshoot reduction function reduces the
output voltage overshoot, energy is dissipated on the
low-side MOSFET, causing additional power loss. The
more frequent the transient event, the more power loss
is dissipated on the low-side MOSFET. The MOSFET may
face severe thermal stress when transient events happen
at a high repetitive rate. User discretion is advised when
this function is enabled.
Key Component Selection
R_BIAS
The ISL62881 uses a resistor (1% or better tolerance is
recommended) from the RBIAS pin to GND to establish
highly accurate reference current sources inside the IC.
Using R_BIAS = 147kΩ sets the controller for CPU core
application and using R_bias = 47kΩ sets the controller for
GPU core application. Do not connect any other
components to this pin. Do not connect any capacitor to
the RBIAS pin as it will create instability.
Care should be taken in layout that the resistor is placed
very close to the RBIAS pin and that a good quality
signal ground is connected to the opposite side of the
R_BIAS resistor.
R_is and C_is
As Figures 1 and 2 show, the ISL62881 needs the
R_is - C_is network across the ISUM+ and the ISUM- pins to
stabilize the droop amplifier. The preferred values are
R_is = 82.5Ω and C_is = 0.01µF. Slight deviations from the
recommended values are acceptable. Large deviations
may result in instability.
Inductor DCR Current-Sensing Network
PHASE
RSUM
ISUM+
L
DCR
RNTCS
RP
RNTC
+
CN VCN
-
RI ^ISUM-
IO
FIGURE 13. DCR CURRENT-SENSING NETWORK
Figure 13 shows the inductor DCR current-sensing
network for a 2-phase solution. An inductor current flows
through the DCR and creates a voltage drop. The
inductor has a resistors in R_sum connected to the phase-
node-side pad and a PCB trace connected to the output-
side pad to accurately sense the inductor current by
sensing the DCR voltage drop. The sensed current
information is fed to the NTC network (consisting of
R_ntcs, R_ntc and R_p) and capacitor C_n. R_ntc is a negative
temperature coefficient (NTC) thermistor, used to
temperature-compensate the inductor DCR change. The
inductor current information is presented to the capacitor
C_n. Equations 7 through 11 describe the frequency-
17
FN6924.0
October 26, 2009