ISL6534 Datasheet, PDF (12/26 Pages) Intersil Corporation – Dual PWM with Linear

ISL6534
VOUT1 (INDEPENDENT OR DDR MODE)
COMP1
VOUT1
R5
FB1
EA
R6
0.6V
FIGURE 5. RESISTOR DIVIDER FOR VOUT1 (DDR OR
INDEPENDENT MODE)
Figure 5 shows the resistors for VOUT1.
Case 1 is the usual case, where R5 and R6 divide VOUT1
down to match the 0.6V internal reference. VOUT1 must be
greater than 0.6V; 2 resistors are needed, and their accuracy
directly affect the regulator tolerance.
Case 2 can be used only if VOUT1 equals exactly 0.6V; then
no divider is needed; only one resistor (R5, which is part of
the compensation) is needed, and its accuracy does not
directly affect the output tolerance.
VOUT1 cannot be less than 0.6V.
Case 1 (divide VOUT1):
REF = 0.6V
FB1 = VOUT1*R6/(R5+R6)
Case 2 (no dividers):
REF = 0.6V
FB1 = VOUT1 (no R6)
VOUT2 (INDEPENDENT MODE)
Figure 6 shows the resistors for VOUT2.
Case 1 is the most general case (no restriction on VREF > or
< VOUT2), and the most flexible. Both VREF and the output
are divided down to the same arbitrary reference (in the 0.6V
to 3.3V range for best performance). The advantage is that if
either the VREF or desired output voltage changes going
forward, the only board change needed is the value of 1 or
more resistors. The disadvantage is that since there are two
resistor dividers, both of them add to the error budget of the
regulator output. The total number of resistors used is 4.
Case 2 can be used when VOUT2 is less than VREF. R3 and
R4 divide the reference to match VOUT2. It saves a resistor
(R2); R1 (usually ~1kΩ) is still needed as part of the
compensation, but it doesn’t affect the accuracy of the output.
Three resistors are needed; this is the most typical case.
Case 3 can be used only when VOUT2 is greater than VREF,
which is brought directly into REFIN; then VOUT2 is divided
down to match it. Only two resistors (R1, R2) are needed,
and both affect the accuracy.
Case 4 can be used only if VREF = VOUT2; this case is the
most accurate (since neither has a divider), and only uses
one resistor (R1, as part of the compensation).
VOUT2
R1
COMP2
FB2
EA
R2
VREF (IND)
OR
VOUT1 (DDR)
R3 REFIN
R4
FIGURE 6. RESISTOR DIVIDERS FOR VOUT2 AND REFIN
Case 1 (divide both signals):
REFIN = VREF*R4/(R3+R4)
FB2 = VOUT2*R2/(R1+R2)
Case 2 (divide VREF):
REFIN = VREF*R4/(R3+R4)
FB2 = VOUT2 (no R2)
Case 3 (divide VOUT2):
REFIN = VREF (no R3, R4)
FB2 = VOUT2*R2/(R1+R2)
Case 4 (no dividers)
REFIN = VREF (no R3, R4)
FB2 = VOUT2 (no R2)
VOUT2 (DDR MODE)
The main difference for DDR Mode is that rather than using a
fixed external reference for REFIN, a reference based on
VOUT1 (which is also called VDDQ for DDR) is used
instead. See Figure 6.
Case 1 is again the most general case; Both VOUT1 and the
VOUT2 output are divided down to the same arbitrary
reference (in the 0.6V to 3.3V range for best performance).
The trade-offs are the same as Case 1 for Independent
mode described earlier.
Case 2 can be used when VOUT2 is less than VOUT1,
which is the case for DDR (since VOUT2 = 1/2 VOUT1). It
saves a resistor (R2); R1 is still needed as part of the
compensation, but it doesn’t affect the accuracy of the
output. R3 and R4 divide the VOUT1 by 2 to match VOUT2.
Three resistors are needed, two of which affect the accuracy.
Since the DDR mode almost always uses the divide by two,
no flexibility is lost here; just change the VOUT1 resistor
divider to change VDDQ, and VOUT2 will still track at 1/2 the
value.
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FN9134.1