English
Language : 

ISL62883C Datasheet, PDF (25/43 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs
◁
Rntcs
Rntc
Cn.1
Rp
Rn
OPTIONAL
ISL62883C
ISUM+
Cn.2 Vcn
Ri
ISUM-
transient response waveforms on an actual board. The
recommended range for Cip is 100pF~2000pF. However,
it should be noted that the Rip -Cip branch may distort
the idroop waveform. Instead of being triangular as the
real inductor current, idroop may have sharp spikes,
which may adversely affect idroop average value
detection and therefore may affect OCP accuracy. User
discretion is advised.
Resistor Current-Sensing Network
Phase1 Phase2 Phase3
Rip Cip
L
L
L
OPTIONAL
FIGURE 22. OPTIONAL CIRCUITS FOR RING BACK
REDUCTION
Figure 21 shows the output voltage ring back problem
during load transient response. The load current io has a
fast step change, but the inductor current iL cannot
accurately follow. Instead, iL responds in first order
system fashion due to the nature of current loop. The
ESR and ESL effect of the output capacitors makes the
output voltage Vo dip quickly upon load current change.
However, the controller regulates Vo according to the
droop current idroop, which is a real-time representation
of iL; therefore it pulls Vo back to the level dictated by iL,
causing the ring back problem. This phenomenon is not
observed when the output capacitor have very low ESR
and ESL, such as all ceramic capacitors.
Figure 22 shows two optional circuits for reduction of the
ring back.
Cn is the capacitor used to match the inductor time
constant. It usually takes the parallel of two (or more)
capacitors to get the desired value. Figure 22 shows that
two capacitors Cn.1 and Cn.2 are in parallel. Resistor Rn is
an optional component to reduce the Vo ring back. At
steady state, Cn.1 + Cn.2 provides the desired Cn
capacitance. At the beginning of io change, the effective
capacitance is less because Rn increases the impedance
of the Cn.1 branch. As Figure 19 explains, Vo tends to dip
when Cn is too small, and this effect will reduce the Vo
ring back. This effect is more pronounced when Cn.1 is
much larger than Cn.2. It is also more pronounced when
Rn is bigger. However, the presence of Rn increases the
ripple of the Vn signal if Cn.2 is too small. It is
recommended to keep Cn.2 greater than 2200pF. Rn
value usually is a few ohms. Cn.1, Cn.2 and Rn values
should be determined through tuning the load transient
response waveforms on an actual board.
Rip and Cip form an R-C branch in parallel with Ri,
providing a lower impedance path than Ri at the
beginning of io change. Rip and Cip do not have any
effect at steady state. Through proper selection of Rip
and Cip values, idroop can resemble io rather than iL, and
Vo will not ring back. The recommended value for Rip is
100Ω. Cip should be determined through tuning the load
DCR DCR
DCR
Rsen Rsen Rsen
Rsum
Rsum
Rsum
Ro
Ro
Ro
ISUM+
Vcn
Cn
Ri ISUM-
Io
FIGURE 23. RESISTOR CURRENT-SENSING NETWORK
Figure 23 shows the resistor current-sensing network for
a 2-phase solution. Each inductor has a series
current-sensing resistor Rsen. Rsum and Ro are
connected to the Rsen pads to accurately capture the
inductor current information. The Rsum and Ro resistors
are connected to capacitor Cn. Rsum and Cn form a a
filter for noise attenuation. Equations 25 thru 27 give
VCn(s) expression
VCn(s)
=
-R----s---e---n--
N
×
Io(s)
×
ARsen(s)
(EQ.25)
ARsen(s)
=
----------1------------
1
+
------s------
ωsns
(EQ.26)
ωRsen
=
--------------1--------------
R-----s---u---m---
N
×
Cn
(EQ.27)
Transfer function ARsen(s) always has unity gain at DC.
Current-sensing resistor Rsen value will not have
significant variation over-temperature, so there is no
need for the NTC network.
The recommended values are Rsum = 1kΩ and
Cn = 5600pF.
Overcurrent Protection
Refer to Equation 1 on page 17 and Figures 17, 21 and
23; resistor Ri sets the droop current Idroop. Table 4 on
page 21 shows the internal OCP threshold. It is
recommended to design Idroop without using the Rcomp
resistor.
25
FN7557.1
March 18, 2010
 ▷