sc2434 Semtech Corporation, sc2434 Datasheet - Page 9

• Manufacturer Part Number: sc2434
• Description: Sc2434 Triphase Current Mode Controller With Power Good
• Manufacturer: Semtech Corporation

be caused by layout inductances, may alter the PWM
comparator trip point. The value of R
adjusted to compensate for such parasitic effects.
It must be noted that the current amplifier gain is quite
precise, with greater than 80dB of Common Mode
Rejection Ratio (CMRR). Thus the droop accuracy is
primarily based upon external components tolerances. By
employing 1% current sensing element with very low
temperature coefficient, this topology is proved to be the
best comparing the schemes of using R
using inductor winding resistance sensing. The accurate
drooping translates into minimum amount output bulk
capacitor needed to meet the voltage regulation specifica-
tions and the least system cost.
Programming The DC Level Of The Output Voltage
Kirchoff’s current law can be applied to the error
amplifier’s inverting input (see Fig. 1) to calculate R
the DC level setting resistor. For given output voltage set
point and VID setting, the resistance can be calculated
by:
where N
highest set point (VID=00000). For example, when VID
[4:1]=00100, N
output voltage and, as a first approximation, it is equal to
1..7V. Again, V
The final value of R
experimentally after the droop resistor has been chosen.
Control Loop Compensation
The current mode control yields a power supply easy to
compensate because the power stage has first order (single
pole) behavior. The SC2434 provides internal slope
compensation to avoid sub harmonic oscillation of the
current loop. The added ramp signal has 300mV peak-to-
peak amplitude and the ramp frequency is as same as
the oscillator frequency.
As depicted in Fig. 4, the gain for the voltage feedback
loop can be expressed as a product of the power stage
gain and the compensator gain:
POWER MANAGEMENT
Applications Information (Cont.)
R os
2005 Semtech Corp.
V set V bg
DAC_STEP
R FB
is the number of VID steps down from the
BG
DAC_STEP
= Precision Reference Voltage = 1.5V.
V eo
OS
R drp
= 4. V
may need to be fine tuned
V bg
V bg
EO
N DAC_STEP I DAC_LSB
is the error amplifier
DRP
dson
may have to be
sensing and
.
OS
,
9
Fig. 4 - Loop gain and compensation of the current mode
troller.
where C
error amplifier output and the inverting input with a value
of 11pF.
The power stage transfer function under continuous
conduction mode can be approximated by:
where G
The power stage has an ESR zero, a dominant pole at
low frequency, and a pair of complex pole located at one
half of the switching frequency. The parameter used here
are defined as below:
The PWM gain is defined as:
H p_ccm s R
Vout
Ccomp
F
R = load resistance
R
C = output bulk capacitance
SW
C
opam
(
PWM
= ESR of output bulk capacitor
0
= switching frequency
,
Ccomp
is the equivalent internal capacitor across the
G p w m
is the low frequency gain of the power stage.
Loop s R
)
Rdrp
+
-
POWER
STAGE
Copam
Err_A mp
Rcomp
G pwm
(
,
)
.
(
Verror
1 s R . C .
Vin/( VR*N phas e)
H p_ccm s R ,
Rdrp/R fb
R
R ! N
s e n s e · G C A
1/(R*C)
Power St age
1/(R*C)
) 1 1.5
(
Loop G ain
.
1
p h a s e
Compensator
) H c s ( )
-1
.
s C . R c
1/(E SRC)
-1
Pol e
.
π F s
.
www.semtech.com
.
SC2434
s
0dB
Fsw/2
Fsw/2
Zero
π F s
-2
.
-2
s
con-
2