ISL6336D Datasheet, PDF (15/30 Pages) Intersil Corporation – VR11.1, 6-Phase PWM Controller with Phase Dropping,Droop Disabled and Load Current Monitoring Features

ISL6336D
The internal circuitry, shown in Figures 4 and 5, represents one
channel of an N-channel converter. This circuitry is repeated for
each channel in the converter, but may not be active depending
on the status of the PWM2, PWM3 and PWM4 pins, as described
in “PWM and PSI# Operation” on page 14. The input bias current
of the current sensing amplifier is typically 60nA; less than 5kΩ
input impedance is preferred to minimized the offset error.
INDUCTOR DCR SENSING
An inductor’s winding is characteristic of a distributed resistance,
as measured by the DCR (Direct Current Resistance) 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.
V_IN
ISL6596
PWM(n)
ILs
L
DCR
INDUCTOR
V_L
V_C(s)
R
C
V_OUT
C_OUT
ISL6336D INTERNAL CIRCUIT
I_n
R_ISEN(n)
CURRENT
SENSE
ISENn-
+
-
ISENn+
C_T
ISEN _{= IL R---}^D_--I--S-^C_----E-^R_---N---
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

------^L-------
DCR
+
1_

DCR

I_L
--------------------_--_s-----_---_R----_C------+-----₁----_-------------------
(EQ. 5)
If the R-C network components are selected such that the RC
time constant (= R*C) matches the inductor time constant
(= 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 ISEN+ pin, I_SEN, is proportional to
the inductor current.
Because of the internal filter at ISEN- pin, one capacitor, C_T, is
needed to match the time delay between the ISEN- and ISEN+
signals. Select the proper C_T to keep the time constant of R_ISEN
and C_T (R_ISEN x C_T) close to 27ns.
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

---^D----^C-----^R-----
R_ISEN
(EQ. 6)
The inductor DCR value will increase as the temperature increases.
Therefore, the sensed current will increase as the temperature of
the current sense element increases. In order to compensate the
temperature effect on the sensed current signal, a Positive
Temperature Coefficient (PTC) resistor can be selected for the
sense resistor R_ISEN, or the integrated temperature compensation
function of ISL6336D should be utilized. The integrated
temperature compensation function is described in “External
Temperature Compensation” on page 24.
RESISTIVE SENSING
For accurate current sense, 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 is more
accurate, but reduces overall converter efficiency due to the
additional power loss on the current sense element R_SENSE.
IL
L
ISL6336D INTERNAL CIRCUIT
R
R_SEN ESL
R_SENSE
V_R
V_C(s)
V_OUT
C_OUT
I_n
C
R_ISEN(n)
CURRENT
SENSE
ISENn-
+
-
ISENn+
C_T
I_SEN
=
I_L
-^R-----^S----^E----^N---
R_ISEN
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.
Submit Document Feedback ₁₅
FN8320.0
October 6, 2014