ISL95870 Datasheet, PDF (19/29 Pages) Intersil Corporation – PWM DC/DC Controller with VID Inputs for Portable GPU Core-Voltage Regulator

ISL95870, ISL95870A, ISL95870B
High Output Voltage Programming
The ISL95870 has a fixed 0.5V reference voltage
(V_SREF). For high output voltage application, the resistor
divider consisting of R_FB and R_OFS requires large ratio
(R_FB :R_OFS = 9:1 for 5V output). The FB pin with large
ratio resistor divider is noise sensitive and the PCB layout
should be carefully routed. It is recommended to use
small value resistor divider such as R_FB=1kΩ.
In general the ISL95870A and ISL95870B have much
better jitter performance than the ISL95870 when the
output voltage is in the range of 3.3V to 5V, particularly
in DCM. This is because V_SREF voltage can be set to 1.5V
and a smaller ratio resistor divider can be used. This
makes the singal to noise ratio at FB pin much better. So
for 3.3V to 5V output, the ISL95870A and ISL95870B are
recommended with V_SREF set to 1.5V.
R4 Modulator
The R⁴ modulator is an evolutionary step in R³
technology. Like R³, the R⁴ modulator allows variable
frequency in response to load transients and maintains
the benefits of current-mode hysteretic controllers.
However, in addition, the R⁴ modulator reduces regulator
output impedance and uses accurate referencing to
eliminate the need for a high-gain voltage amplifier in
the compensation loop. The result is a topology that can
be tuned to voltage-mode hysteretic transient speed
while maintaining a linear control model and removes the
need for any compensation. This greatly simplifies the
regulator design for customers and reduces external
component cost.
Stability
The removal of compensation derives from the R⁴
modulator’s lack of need for high DC gain. In traditional
architectures, high DC gain is achieved with an
integrator in the voltage loop. The integrator introduces
a pole in the open-loop transfer function at low
frequencies. That, combined with the double-pole from
the output L/C filter, creates a three pole system that
must be compensated to maintain stability.
Classic control theory requires a single-pole transition
through unity gain to ensure a stable system.
Current-mode architectures (includes peak, peak-valley,
current-mode hysteretic, R³ and R⁴) generate a zero at
or near the L/C resonant point, effectively canceling one
of the system’s poles. The system still contains two
poles, one of which must be canceled with a zero before
unity gain crossover to achieve stability. Compensation
components are added to introduce the necessary zero.
COMPENSATION TO COUNTER
INTEGRATOR POLE
INTEGRATOR
FOR HIGH DC GAIN
VV^OUT
V COMP
V_DAC
FIGURE 12. INTEGRATOR ERROR-AMPLIFIER
CONFIGURATION
R3 LOOP GAIN (dB)
INTEGRATOR POLE
p1
L/C DOUBLE-POLE
p2
p3
-20dB CROSSOVER
REQUIRED FOR STABILITY
CURRENT-MODE
ZERO _z1
COMPENSATOR TO
ADD z2 IS NEEDED
-20dB/dec
f (Hz)
FIGURE 13. UNCOMPENSATED INTEGRATOR
OPEN-LOOP RESPONSE
Figure 12 illustrates the classic integrator configuration
for a voltage loop error-amplifier. While the integrator
provides the high DC gain required for accurate
regulation in traditional technologies, it also introduces a
low-frequency pole into the control loop. Figure 13 shows
the open-loop response that results from the addition of
an integrating capacitor in the voltage loop. The
compensation components found in Figure 12 are
necessary to achieve stability.
Because R⁴ does not require a high-gain voltage loop,
the integrator can be removed, reducing the number of
inherent poles in the loop to two. The current-mode zero
continues to cancel one of the poles, ensuring a
single-pole crossover for a wide range of output filter
choices. The result is a stable system with no need for
compensation components or complex equations to
properly tune the stability.
R2
V_OUT
R1
V_COMP
V_DAC
FIGURE 14. NON-INTEGRATED R4 ERROR-AMPLIFIER
CONFIGURATION
19
FN6899.0
December 22, 2009