ISL68200 Datasheet, PDF (16/32 Pages) Intersil Corporation – Single-Phase R4 Digital Hybrid PWM Controller with Integrated Driver

ISL68200
As shown in Table 7, 1 step is 2^-7 = 7.8125mV; some selections
are higher than 1 step from adjacent codes. However, the
resolution is ±7.8125mV around the popular voltage regulation
points, as in Table 3 on page 11, for fine tune purpose. For finer
than 7.8125mV tuning, a large ratio resistor divider can be
placed on the VSEN pin between the output (V_OUT) and RGND for
positive offset or V_CC for negative offset, as in Figure 8.
V_OUT
VSEN
+
-
V_OUT
V_CC
VSEN
+
-
RGND
A. V_OUT HIGHER THAN DAC
B. V_OUT LOWER THAN DAC
FIGURE 8. EXTERNAL PROGRAMMABLE REGULATION
Current Sensing
The ISL68200 supports inductor DCR sensing, or resistive
sensing techniques, and senses current continuously for fast
response. The current sense amplifier uses the CSEN and CSRTN
inputs to reproduce a signal proportional to the inductor current,
I_L. The sense current, I_SEN, is proportional to the inductor current
and is used for current reporting and overcurrent protection.
The input bias current of the current sensing amplifier is typically
10s of nA; less than 15kΩ input impedance connected to CSEN
pin is preferred to minimize the offset error, i.e., use a larger C
value (select 0.22µF to 1µF instead of 0.1µF when needed). In
addition, the current sensing gain resistor connected to CSRTN
pin should be within 40Ωto 3.5kΩ.
INDUCTOR DCR SENSING
An inductor’s winding is characteristic of a distributed resistance,
as measured by the DCR (Direct Current Resistance) parameter.
A simple R-C network across the inductor extracts the DCR
voltage, as shown in Figure 9.
The voltage on the capacitor V_C, can be shown to be proportional
to the inductor current I_{L ,} as in 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 (= 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. 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 CSRTN pin, I_SEN,
is proportional to the inductor current.
Equation 6 shows that the ratio of the inductor 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--
(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,
the integrated temperature compensation function of ISL68200
should be utilized. The integrated temperature compensation
function is described in “Thermal Monitoring and Compensation”
on page 17.
DRIVER
ISL68200
INTERNAL CIRCUIT
I_OUT
CURRENT
SENSE
V_IN
ILs
L
DCR
INDUCTOR
V_L
V_OUT
C_OUT
PLACE THESE IN CLOSE
PROXIMITY TO ISL68200
V_C(s)
OPTIONAL
R
C
R_ISEN
CSEN
+
-
CSRTN
ISEN _{= IL R----}^D_-I--S-^C_----E-^R_---N---
FIGURE 9. DCR SENSING CONFIGURATION
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 10). This technique, however, reduces
overall converter efficiency due to the additional power loss on the
current sense element R_SENSE.
IL
ISL68200
INTERNAL CIRCUIT
I_OUT
L
R_SEN ESL
V_OUT
R_SENSE
V_R
C_OUT
PLACE THESE IN CLOSE
PROXIMITY TO ISL68200
R _VC(s)
OPTIONAL
CURRENT
SENSE
CSEN
+
-
C
R_ISEN
I_{SEN = IL R-}^R_----I-^S_-S---^E_-E---^N_-N--
CSRTN
FIGURE 10. SENSE RESISTOR IN SERIES WITH INDUCTORS
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FN8705.1
March 7, 2016