ISL6742 Datasheet, PDF (15/18 Pages) Intersil Corporation – Advanced Double-Ended PWM Controller

ISL6742
If ΔV_CS is greater than or equal to Ve, then no additional slope
compensation is needed and R_CS becomes Equation 22:
R_CS
=
------------------------------------------------------------^N----^C----^T-------------------------------------------------------------
N_-----S--
N_P
⋅
⎛
⎜
⎝
I_O
+
D_-----t_--S----W---
2L_O
⋅
⎛
⎝⎜ ^{V I N}
⋅
N_-----S--
N_P
–
⎞⎞
V_{O⎠⎟}
⎟
⎠
+
V_{-----I--N-----}⋅_---D_-----t_--S----W---
L_m
(EQ. 22)
If ΔV_CS is less than V_e, then Equation 19 is still valid for the
value of R_CS, but the amount of slope compensation added
by the external ramp must be reduced by ΔV_CS.
Adding slope compensation is accomplished in the ISL6742
using an external buffer and the CT signal. A typical
application sums the buffered CT signal with the current
sense feedback and applies the result to the CS pin as
shown in Figure 14.
1 _VREF
16
2
ISL6742 ¹⁵
3
14
4 _CT
13
5
12
R9
6
11
7 _CS
10
8
9
R6
R_CS
C4
CT
FIGURE 14. ADDING SLOPE COMPENSATION
Assuming the designer has selected values for the RC filter
(R6 and C4) placed on the CS pin, the value of R9 required
to add the appropriate external ramp can be found by
superposition.
V_e – ΔV_CS
=
-²----^D------^â‹---^R-----⁶---
R6 + R9
V
(EQ. 23)
Rearranging to solve for R9 yields:
R9
=
-⁽--²----^D------–----^V----^e-----+-----^Δ----^V----^C----^S----⁾----^â‹---^R----⁶--
V_e – ΔV_CS
Ω
(EQ. 24)
The value of R_CS determined in Equation 19 must be
rescaled so that the current sense signal presented at the
CS pin is that predicted by Equation 17. The divider created
by R6 and R9 makes this necessary.
R′_CS
=
R_-----6_-----+-----R_-----9_--
R9
⋅
R_CS
(EQ. 25)
Example:
V_IN = 280V
V_O = 12V
L_O = 2.0µH
N_P/N_S = 20
Lm = 2mH
I_O = 55A
Oscillator Frequency, F_SW = 400kHz
Duty Cycle, D = 85.7%
N_CT = 50
R6 = 499Ω
Solve for the current sense resistor, R_CS, using Equation 19.
R_CS = 15.1Ω.
Determine the amount of voltage, V_e, that must be added to
the current feedback signal using Equation 16.
V_e = 153mV
Next, determine the effect of the magnetizing current from
Equation 21.
ΔV_CS = 91mV
Using Equation 24, solve for the summing resistor, R9, from
CT to CS.
R9 = 13.2kΩ
Determine the new value of R_CS, R’_CS, using Equation 25.
R’_CS = 15.7Ω
Additional slope compensation may be considered for
design margin. This discussion determines the minimum
external ramp that is required. The buffer transistor used to
create the external ramp from CT should have a sufficiently
high gain (>200) so as to minimize the required base
current. Whatever base current is required reduces the
charging current into CT and will reduce the oscillator
frequency.
Parallel Operation
Parallel operation of converters using the ISL6742 may be
accomplished using the average current signal, IOUT. IOUT
provides a very accurate representation of the output current
and may be used for active current sharing with many
sharing techniques commonly used including master-slave
and average current sharing methods.
Since IOUT represents the average inductor current (CCM
operation), sharing errors introduced by techniques using
peak inductor current are reduced. In particular, the current
sharing error introduced by mismatched switching
frequencies is eliminated.
15
FN9183.2
October 31, 2008