sc2516 Semtech Corporation, sc2516 Datasheet - Page 9

• Manufacturer Part Number: sc2516
• Description: Sc2516 Complete Three-in-one Ddr Power Solution With Bf_cut
• Manufacturer: Semtech Corporation

Compensation design of the VDDQ Channel
Compensation design of the VDDQ Channel
Fig. 1. SC2516 small signal model.
Compensation design of the VDDQ Channel
Compensation design of the VDDQ Channel
Compensation design of the VDDQ Channel
The control model of SC2516 VDDQ and GMCH section
can be depicted in Fig. 1. This model can also be used in
Spice kind of simulator to generate loop gain Bode plots.
The bandgap reference is 1.25 V and trimmed to +/-1%
accuracy. The desired output voltage can be achieved
by setting the resistive divider network, R1 and R2.
The error amplifier is transconductance type with fixed
gain of:
The compensation network includes a resistor and a ca-
pacitor in series, which terminates from the output of
the error amplifier to the ground. The PWM gain is inver-
sion of the ramp amplitude, and this gain is given by:
where the ramp amplitude (peak-to-peak) is 0.55 volts .
The total control loop-gain can then be derived as
follows:
where
T s ( ) T o
POWER MANAGEMENT
Applications Information (Cont.)
© 2005 Semtech Corp.
1.25Vdc
Vbg
T
o
.
:=
1 s R . C .
G
s R . C .
m
⋅
EA
G
pwm
G pwm
G
.
m
1 s R c C o
⋅
V
R
C
:=
Gpwm
in
.
0.001 A ⋅
R ⋅
⋅
V
Vin
⎛
⎜
⎝
V ramp
.
V
V
bg
o
1
1 s R c
⎞
⎠
R o
L
.
L
.
s
C o
2
L . C o
.
Rc
Co
.
Ro
1
R c
R o
R1
R2
9
The task here is to properly choose the compensation
network for a nicely shaped loop-gain Bode plot. The
following design procedures are recommended to accom-
plish the goal:
(1) Calculate the corner frequency of the output filter:
(2) Calculate the ESR zero frequency of the output filter
capacitor:
(3) Check that the ESR zero frequency is not too high.
If this condition is not met, the compensation structure
may not provide loop stability. The solution is to add
some electrolytic capacitors to the output capacitor bank
to correct the output filter corner frequency and the ESR
zero frequency. In some cases, the filter inductance may
also need to be adjusted to shift the filter corner
frequency. It is not recommended to use only high fre-
quency multi-layer ceramic capacitors for output filter.
(4) Choose the loop gain cross over frequency (0 dB fre-
quency). It is recommended that the crossover frequency
is always less than one fifth of the switching frequency :
If the transient specification is not stringent, it is better
to choose a crossover frequency that is less than one
tenth of the switching frequency for good noise immunity.
The resistor in the compensation network can then be
calculated as:
when
F
R
o
:=
<
G
F
pwm
esr
<
⋅
1
V
F
F
F
F
F
in
x_over
esr
o
esr
x_over
⋅
G
:=
:=
<
m
2 π ⋅ L C
⋅
F
2 π ⋅ R
⎛
⎜
⎝
≤
5
⋅
sw
F
F
F
⋅
1
esr
5
o
1
sw
⋅
c
⋅
⎞
⎠
C
o
2
⋅
o
⎛
⎜
⎝
F
F
x_over
esr
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SC2516
⎞
⎠
⋅
⎛
⎜
⎝
V
V
bg
o
⎞
⎠