ISL6398 Datasheet, PDF (18/57 Pages) Intersil Corporation – Programmable soft-start rate and DVID rate

ISL6398
Current Sensing
The ISL6398 senses current continuously for fast response. The
ISL6398 supports inductor DCR sensing, or resistive sensing
techniques. The associated channel current sense amplifier uses
the ISEN inputs to reproduce a signal proportional to the inductor
current, I_L. The sense current, I_SEN, is proportional to the inductor
current. The sensed current is used for current balance, load-line
regulation, and overcurrent protection.
The internal circuitry, shown in Figures 4 and 5, represents one
channel of the VR output, respectively. The ISEN± circuitry is
repeated for each channel, but may not be active depending on
the status of the PWM[6:2] pins, as described in “PWM
Operation” on page 17. The input bias current of the current
sensing amplifier is typically 25nA; less than 8k input
impedance is preferred to minimized the offset error, i.e., a larger
C value as needed.
INDUCTOR DCR SENSING
An inductor’s winding is characteristic of a distributed resistance,
as measured by the Direct Current Resistance (DCR) parameter.
Consider the inductor DCR as a separate lumped quantity, as
shown in Figure 4. The channel current I_L, flowing through the
inductor, will also pass through the DCR. Equation 4 shows the
S-domain equivalent voltage across the inductor V_L.
V_Ls = I_L  s  L + DCR
(EQ. 4)
A simple R-C network across the inductor extracts the DCR
voltage, as shown in Figure 4.
ILs
ISL6398
L
DCR
INDUCTOR
V_L
V_OUT
C_OUT
V_C(s)
I_n
R
C
CURRENT
SENSE
+
-
I_{SEN = IL} ^D_-----^CR_-----S^R_----E--^_--T-⁶_---⁴_--
R_ISEN(n)
ISEN-(n)
ISEN+(n)
RSET
C_T, Optional
FIGURE 4. DCR SENSING CONFIGURATION
The voltage on the capacitor V_C, can be shown to be proportional
to the channel current I_L(see Equation 5).
V_Cs = -^--^s-----^---^{D----------C-L}-------_^-R----_s-----+-_---_R-¹---_C-^----^-+--^--^D-₁----_^C-----^R------^---^I--^L---^
(EQ. 5)
If the R-C network components are selected such that the RC time
constant matches the inductor time constant (R*C = L/DCR), the
voltage across the capacitor V_C is equal to the voltage drop across
the DCR, i.e., proportional to the channel current.
With the internal low-offset current amplifier, the capacitor
voltage V_C is replicated across the sense resistor R_ISEN.
Therefore, the current out of the ISENS+ pin, I_SEN, is proportional
to the inductor current.
Equation 6 shows that the ratio of the channel current to the
sensed current, I_SEN, is driven by the value of the sense resistor
and the DCR of the inductor.
I_SEN = I_L  _R---^D--_I--_S^C----_E-^R---_N-- = I_L ^D-----^CR-----_S^R----_E--^--_T-⁶---⁴--
(EQ. 6)
RESISTIVE SENSING
For more accurate current sensing, a dedicated current-sense
resistor R_SENSE in series with each output inductor can serve as the
current sense element (see Figure 5). This technique however
reduces overall converter efficiency due to the additional power loss
on the current sense element R_SENSE.
IL
ISL6398
L
R
R_SEN ESL
R_SENSE
V_R
V_C(s)
V_OUT
C_OUT
C
I_n
CURRENT
SENSE
+ _RISEN(n)
-
I_{SEN = IL} ^R_-----^S_-R---^E_--S--^N_--E---^_-T--⁶_----⁴_-
ISEN-(n)
ISEN+(n)
RSET
C_T, Optional
FIGURE 5. SENSE RESISTOR IN SERIES WITH INDUCTORS
A current sensing resistor has a distributed parasitic inductance,
known as ESL (equivalent series inductance, typically less than
1nH) parameter. Consider the ESL as a separate lumped
quantity, as shown in Figure 5. The channel current I_L, flowing
through the inductor, will also pass through the ESL. Equation 7
shows the s-domain equivalent voltage across the resistor V_R.
V_Rs = I_L  s  ESL + R_SEN
(EQ. 7)
A simple R-C network across the current sense resistor extracts
the R_SEN voltage, as shown in Figure 5.
The voltage on the capacitor V_C, can be shown to be proportional
to the channel current I_L (see Equation 8).
V_Cs = -^--^s-----^---^R----E-----S----^S----^E-----^L-_--^N--_s------+_---_R--¹---_C^----^-+--^---^R1----^S_----^E----^N-----^---^I--^L---^-
(EQ. 8)
Submit Document Feedback ₁₈
FN8575.1
August 13, 2015