ISL6312A_15 Datasheet, PDF (13/36 Pages) Intersil Corporation – Four-Phase Buck PWM Controller with Integrated MOSFET Drivers for Intel VR10,VR11, and AMD Applications

ISL6312A
balance method is illustrated in Figure 3, with error
correction for Channel 1 represented. The cycle average
current, I_AVG, is compared with the Channel 1 sample, I₁, to
create an error signal I_ER (see Figure 3).
V_COMP
+
-
FILTER f(s)
MODULATOR
RAMP
WAVEFORM
PWM1
+
-
TO GATE
CONTROL
LOGIC
I_ER
I_AVG
-
¸N
+
I₄
S
I₃
I₂
I₁
NOTE: Channel 3 and 4 are optional.
FIGURE 3. CHANNEL-1 PWM FUNCTION AND CURRENT
BALANCE ADJUSTMENT
The filtered error signal modifies the pulse width
commanded by V_COMP to correct any unbalance and force
I_ER toward zero. The same method for error signal
correction is applied to each active channel.
PWM
SWITCHING PERIOD
I_L
I_SEN
TIME
FIGURE 4. CONTINUOUS CURRENT SAMPLING
Continuous Current Sampling
In order to realize proper current balance, the currents in
each channel are sensed continuously every switching
cycle. During this time the current sense amplifier uses the
ISEN inputs to reproduce a signal proportional to the
inductor current, I_L. This sensed current, I_SEN, is simply a
scaled version of the inductor current.
The ISL6312A supports inductor DCR current sensing to
continuously sense each channel’s current for channel current
balance. The internal circuitry, shown in Figure 5 represents
Channel N of an N-channel converter. This circuitry is repeated
for each channel in the converter, but may not be active
depending on how many channels are operating.
MOSFET
DRIVER
V_IN
UGATE(n)
LGATE(n)
ISL6312A INTERNAL CIRCUIT
IL
L
DCR
INDUCTOR
V_L(s)
V_C(s)
R₁
C₁
R_2*
V_OUT
C_OUT
I_n
SAMPLE
+
-
I_SEN
V_C(s)
R_ISEN
ISEN-(n)
ISEN+(n)
*R₂ is OPTIONAL
FIGURE 5. INDUCTOR DCR CURRENT SENSING
CONFIGURATION
Inductor windings have a characteristic distributed
resistance or DCR (Direct Current Resistance). For
simplicity, the inductor DCR is considered as a separate
lumped quantity, as shown in Figure 5. The channel current
I_L, flowing through the inductor, passes through the DCR.
Equation 3 shows the s-domain equivalent voltage, V_L,
across the inductor.
V_Ls = I_L  s  L + DCR
(EQ. 3)
A simple R-C network across the inductor (R₁ and C)
extracts the DCR voltage, as shown in Figure 5. The voltage
across the sense capacitor, V_C, can be shown to be
proportional to the channel current I_L, shown in Equation 4.
V_Cs = _---_s--^--_-^D---s--_R-----^C-----₁----^LR----_----_C-+-----+¹---^--₁----_  DCR  I_L
(EQ. 4)
In some cases it may be necessary to use a resistor divider
R-C network to sense the current through the inductor. This
can be accomplished by placing a second resistor, R₂,
across the sense capacitor. In these cases the voltage
across the sense capacitor, V_C, becomes proportional to the
channel current I_L, and the resistor divider ratio, K.
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FN9290.6
January 22, 2015