ISL78220 Datasheet, PDF (17/22 Pages) Intersil Corporation – 6-Phase Interleaved Boost PWM Controller with Light Load Efficiency Enhancement

ISL78220
Current Sensing
The ISL78220 senses the current continuously for fast response.
It supports both sense resistor and inductor DCR current sensing
methods. The sensed current for each active channel will be used
for loop control, phase current balance, individual channel
overcurrent protection and total average current protection. The
internal circuitry, shown in Figures 20 and 21, represents a single
channel. This circuitry is repeated for each channel, but may not
be active depending on the status of the PWM3, PWM4, PWM5,
and PWM6 pin voltage.
Peak current mode control is implemented by feeding back the
current output of the current sense amplifier (CSA) to the
regulator control loop. Individual channel peak current limit is
implemented by comparing the CSA output current with 160µA.
When the peak current limit comparator is tripped, the PWM
on-pulse is terminated and the IC is latched off.
Sense Resistor Current Sensing
A sense resistor can be placed in series with the power inductor.
As shown in Figure 20, The ISL78220 acquires the channel
current information by sensing the voltage signal across the
sense resistor. Because the voltage on both the positive input
and the negative input of CSA are forced to be equal, the voltage
across R_SET is equivalent to the voltage drop across the R_SEN
resistor. The resulting current into the ISENxP pin is proportional
to the channel current, I_L. Equation 5 for I_SEN is derived where I_L
is the channel current:
I_SEN
=
I_L ⋅
-R----^S----^E----^N---
R_ISET
(EQ. 5)
VIN
RSEN
L
ISEN
RSET
VOUT
ISEN
CSA
ISL78220 INTERNAL CIRCUITS
ISEN(n)P
ISEN(n)N
SENSE RESISTOR
CURRENT SENSING
FIGURE 20. SENSE RESISTOR CURRENT SENSING
Inductor DCR Sensing
An inductor’s winding is characteristic of a distributed resistance
as measured by the DCR (Direct Current Resistance) parameter.
VIN
DCR
C
IL
L
R
VOUT
ISEN
RSET
INDUCTOR DCR
CURRENT SENSING
ISEN
CSA
ISEN(n)P
ISEN(n)N
ISL78220 INTERNAL CIRCUITS
FIGURE 21. INDUCTOR DCR CURRENT SENSING
Consider the inductor DCR as a separate lumped quantity, as
shown in Figure 21. The channel current I_L, flowing through the
inductor, will also pass through the DCR. Equation 6 shows the
S-domain equivalent voltage across the inductor V_L.
V_L = I_L ⋅ (s ⋅ L + DCR)
(EQ. 6)
A simple R-C network across the inductor extracts the DCR
voltage, as shown in Figure 21.
The voltage on the capacitor V_C, can be shown to be proportional
to the channel current I_L, see Equation 7.
V_C
=
⎛
⎝
s
⋅
-----^L------
DCR
+
1_⎠⎞
⋅
(DCR
⋅
I_L)
----------------------------------------------------------------
(s ⋅ RC + 1)
(EQ. 7)
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 differential current sense amplifier,
the capacitor voltage V_C is replicated across the sense resistor
R_SET. Therefore the current flows into the ISENxP pin is
proportional to the inductor current. Equation 8 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_SET
(EQ. 8)
17
FN7688.0
December 15, 2011