ISL78268 Datasheet, PDF (24/33 Pages) Intersil Corporation – Integrated 2A sourcing

ISL78268
resulting current into the ISEN(n)P pin is proportional (scaled) to
the current in R_SEN(n). Equation 4 is derived as:
I_{SENn} = I_{RSENn}  ^R-R----S-_S---^EE----^NT---_-^-_n-ⁿ---_
(EQ. 4)
Where R_SET(n) is the sum of R_SET(n)A and R_SET(n)B in Figure 44.
ISEN(n)P and ISEN(n)N have equal bias current (112µA typ)
therefore, the resistors R_BIAS(n) and R_SET(n) should be matched
to prevent offset.
To prevent noise injection from switching currents, it is
recommended to place a filter capacitor in between the R_SET
resistors. Typically, 220pF ceramic capacitor is used when the
R_SET(n) is 665Ω.
CIN
UG
VIN
ISEN1 + 112µA _ISEN1P
+
RSEN1
-
RSET1A
RBIAS1A
RSET1B
RBIAS1B
AV = 1
ISEN1N
CSA1
111122µµAA
112µA
ISEN1
LG
L
VOUT
+
RSEN2 _-
COUT
IL
ISEN2+112µA ISEN2P
RSET2A
RBIAS2A
RSET2B
RBIAS2B
AV = 1
CSA2
ISEN2N
111122µµAA
112µA
ISEN2
ISL78268 INTERNAL CIRCUITS
FIGURE 44. SENSE RESISTOR CURRENT SENSING
INDUCTOR DCR SENSING
An inductor has a distributed resistance as measured by the DCR
(Direct Current Resistance) parameter.
The inductor DCR can be modeled as a lumped quantity, as
shown in Figure 45, Equation 5 shows the S-domain equivalent
voltage across the inductor V_L.
V_L = I_L  s  L + DCR
(EQ. 5)
A simple R-C network across the inductor can extract the DCR
voltage, as shown in Figure 45.
The voltage on the capacitor V_CDCRS, can be shown to be
proportional to the channel current I_L, see Equation 6.
V_CDCRS
=
----------^---^s----^---^{D----------CL-------R}--------+-----¹---^-----^---^--^D----^C----^R------^---^I--^L---^----------
R_DCRS



-----¹-------
R_SET
+
s

C_D
C

R S
+
1
(EQ. 6)
If the C_DCRS is selected so 2**f_SW*C_DCRS is much greater than
1/R_SET, the 1/R_SET will be negligible. Also, if the R-C network
components are selected such that the time constant
(R_DCRS*C_DCRS) matches the inductor time constant (L/DCR),
the voltage across the capacitor V_CDCRS is equal to the voltage
drop across the DCR, i.e., proportional to the inductor current.
With the internal current sense amplifier, the capacitor voltage
V_CDCRS is replicated across the sense resistor R_SET2. Therefore,
the current flow into the ISEN2P pin is also proportional to the
inductor current. Equation 7 shows the relation between sensed
current I_SEN2 and inductor current (I_L) when DCR sensing is used.
I_SEN2
=
I_L

---^D----^C----^R-----
R_SET2
(EQ. 7)
VIN
+
CIN
RSEN1
-
UG
TO
CSA1
LG
VOUT
+
DCR
IL
RDCRS
+
ISEN2P
-
L
-
+
-
CDCRS^RRSBEIAT2S2
AV = 1
ISEN2N
COUT
11122µµAA
CSA2
112µA
ISEN2
ISL78268 INTERNAL CIRCUITS
FIGURE 45. INDUCTOR DCR CURRENT SENSING
Adjustable Slope Compensation
A buck converter operating in peak current mode requires slope
compensation when the duty cycle is larger than 50%. It is
advisable to add slope compensation when the duty cycle is
approximately 30% or more since a transient load step can push
the duty cycle higher than the steady state level. When slope
compensation is too low, the converter can suffer from
subharmonic oscillation, which may result in noise emissions at
half the switching frequency. On the other hand,
overcompensation of the slope may reduce the phase margin.
Therefore, proper design of the slope compensation is needed.
The ISL78268 features adjustable slope compensation by
setting the resistor value R_SLOPE from the SLOPE pin to GND.
Figure 46 shows the block diagram related to slope compensation.
For current mode control, in theory we need the compensation
slope m_SL to be larger than 50% of the inductor current down
ramp slope m_b.
Equation 8 shows the resistor value at SLOPE PIN to create a
compensation ramp.
R_SLOPE
=
---------^L---^x---¹----⁰----⁶---^x---^R----^S----^E---^T--------- 
K x V_{O U T} x R_{S E N} x 1.5
(EQ. 8)
Where K is the selected gain of compensation slope over
inductor down slope. For example, K = 1 gives the R_SLOPE value
generating a compensation slope equal to inductor current down
ramp slope. Theoretically, the K needs to be larger than 0.5 and
in general, more than 1.0 is used in the actual application.
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FN8657.3
December 12, 2014