ISL62884C Datasheet, PDF (19/30 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6™ Mobile CPUs

ISL62884C
ISUM+
Rntcs
Rp
Rntc
Cn.1
Rn
OPTIONAL
+
Cn.2 Vcn
-
Ri ^ISUM-
Rip ^Cip
OPTIONAL
FIGURE 15. OPTIONAL CIRCUITS FOR RING BACK
REDUCTION
C_n 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 15 shows
that two capacitors C_n.1 and C_n.2 are in parallel.
Resistor R_n is an optional component to reduce the V_o
ring back. At steady state, C_n.1 + C_n.2 provides the
desired C_n capacitance. At the beginning of i_o change,
the effective capacitance is less because R_n increases
the impedance of the C_n.1 branch. As Figure 12
explains, V_o tends to dip when C_n is too small, and this
effect will reduce the V_o ring back. This effect is more
pronounced when C_n.1 is much larger than C_n.2. It is
also more pronounced when R_n is bigger. However, the
presence of R_n increases the ripple of the V_n signal if
C_n.2 is too small. It is recommended to keep C_n.2
greater than 2200pF. R_n value usually is a few ohms.
C_n.1, C_n.2 and R_n values should be determined
through tuning the load transient response waveforms
on an actual board.
Resistor Current-Sensing Network
PHASE
L
DCR
RSEN
RSUM
Vcn
ISUM+
Cn
Ri
ISUM-
Io
FIGURE 16. RESISTOR CURRENT-SENSING NETWORK
Figure 16 shows the resistor current-sensing network.
The inductor has a series current-sensing resistor R_sen.
R_sum and is connected to the R_sen pad to accurately
capture the inductor current information. The R_sum feeds
the sensed information to capacitor C_n. R_sum and C_n
form a a filter for noise attenuation. Equations 12
through 14 gives V_Cn(s) expressions:
V_Cn(s) = R_sen × I_o(s) × A_Rsen(s)
(EQ. 12)
A_Rsen(s)
=
-----------¹-----------
1 + -_ω----_s-^s--_n---_s-
ω_Rsen
=
-------------¹---------------
R_sum × C_n
(EQ. 13)
(EQ. 14)
Transfer function A_Rsen(s) always has unity gain at DC.
Current-sensing resistor R_sen value will not have
significant variation over-temperature, so there is no
need for the NTC network.
The recommended values are R_sum = 1kΩ and
C_n = 5600pF.
Overcurrent Protection
Referring to Equation 1 and Figures 9, 10 and 16, resistor
R_i sets the droop current I_droop. Table 3 shows the
internal OCP threshold. It is recommended to design
I_droop without using the R_comp resistor.
For example, the OCP threshold is 60µA. We will design
I_droop to be 50µA at full load, so the OCP trip level is 1.2x
of the full load current.
For inductor DCR sensing, Equation 15 gives the DC
relationship of V_cn(s) and I_o(s).
V_Cn
=
⎛
⎜
⎝
-----------^R-----ⁿ---^t--^c---ⁿ---^e----^t----------
R_ntcnet + R_sum
×
D
⎞
C R⎟
⎠
× I_o
(EQ. 15)
Substitution of Equation 15 into Equation 1 gives
Equation 16:
I_droop
=
--²---
R_i
×
-----------^R-----ⁿ---^t--^c---ⁿ---^e----^t----------
R_ntcnet + R_sum
×
DCR
×
I_o
(EQ. 16)
Therefore:
R_i
=
--------²----^R----ⁿ----^t-^c---ⁿ----^e---^t---^×-----^D-----^C----^R------^×-----^I--^o---------
(R_ntcnet + R_sum) × I_droop
(EQ. 17)
Substitution of Equation 7 and application of the OCP
condition in Equation 17 gives Equation 18:
R_i
=
-----²-----^×-----^(-----RR---------^n-n------^tt----^cc------^ss--------++----------^RR---------^nn-------tt---^c-c------⁾--+----^×-----^R-----R----^p-----^p------×-----^D-----^C-----^R------^×----^I--^o----^m----^a----^x-----
⎛
⎜
⎝
(_---R_{-----n---t--c---s----+-----}R_{----n----t-c----})_---×_-----R_----p--
R_ntcs + R_ntc + R_p
+
⎞
R _{s u m⎠⎟}
×
I_droopmax
(EQ. 18)
where I_omax is the full load current, I_droopmax is the
corresponding droop current. For example, given
R_sum = 1.82kΩ, R_p = 11kΩ, R_ntcs = 2.61kΩ, R_ntc = 10kΩ,
DCR = 19.7mΩ, I_omax = 5A and I_droopmax = 50µA,
Equation 18 gives R_i = 3.01kΩ.
For resistor sensing, Equation 19 gives the DC
relationship of V_cn(s) and I_o(s).
V_Cn = R_sen × I_o
(EQ. 19)
19
FN7591.0
March 16, 2010