sc488mltrt Semtech Corporation, sc488mltrt Datasheet - Page 12

• Manufacturer Part Number: sc488mltrt
• Description: Sc488 Complete Ddr1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

Switching Frequency
Switching frequency determines the trade-off between
size and effi ciency. Higher frequency increases switch-
ing losses in the MOSFETs, since losses are a function of
F*VIN2. Knowing the maximum input voltage and budget
for MOSFET switches usually dictates the fi nal design.
Inductor Ripple Current
Low inductor values result in smaller size, but create high-
er ripple current and are less effi cient because of the high
AC current fl owing in the inductor. Higher inductor values
do reduce the ripple current and are more effi cient, but
are larger and more costly. The selection of the ripple cur-
rent is based on the maximum output current and tends
to be between 20% to 50% of the maximum load current.
Again, cost, size and effi ciency all play a part in the selec-
tion process.
Stability Considerations
Unstable operation shows up in two related but distinctly
different ways: double pulsing and fast-feedback loop in-
stability. Double-pulsing occurs due to noise on the output
or because the ESR is too low, causing insuffi cient voltage
ramp in the output signal. This causes the error amplifi er to
trigger prematurely after the 400ns minimum off-time has
expired. Double-pulsing will result in higher ripple voltage at
the output, but in most cases is harmless. In some cases,
however, double-pulsing can indicate the presence of loop
instability, which is caused by insuffi cient ESR. One simple
way to solve this problem is to add some trace resistance
in the high current output path. A side effect of doing this
is output voltage droop with load. Another way to eliminate
doubling-pulsing is to add a 10pF capacitor across the
upper feedback resistor divider network. This is shown in
Figure 6, by capacitor C4 in the schematic. This capacitance
should be left out until confi rmation that double-pulsing ex-
ists. Adding this capacitance will add a zero in the transfer
function and should eliminate the problem. It is best to
leave a spot on the PCB in case it is needed.
Loop instability can cause oscillations at the output as a
response to line or load transients. These oscillations can
trip the over-voltage protection latch or cause the output
voltage to fall below the tolerance limit.
© 2006 Semtech Corp.
POWER MANAGEMENT
Application Information (Cont.)
12
The best way for checking stability is to apply a zero to
full load transient and observe the output voltage ripple
envelope for overshoot and ringing. Over one cycle of ring-
ing after the initial step is a sign that the ESR should be
increased.
SC488 ESR Requirements
The constant on-time control used in the SC488 regulates
the ripple voltage at the output capacitor. This signal
consists of a term generated by the output ESR of the
capacitor and a term based on the increase in voltage
across the capacitor due to charging and discharging
during the switching cycle. The minimum ESR is set to
generate the required ripple voltage for regulation. For most
applications the minimum ESR ripple voltage is dominated
by PCB layout and the properties of SP or POSCAP type
output capacitors. For applications using ceramic output
capacitors, the absolute minimum ESR must be considered.
If the ESR is low enough the ripple voltage is dominated
by the charging of the output capacitor. This ripple voltage
lags the on-time due to the LC poles and can cause double
pulsing if the phase delay exceeds the off-time of the
converter. Referring to Figure 5 on Page 10, the equation
for the minimum ESR as a function of output capacitance
and switching frequency and duty cycle is:
SC488
ESR
PGND
VDDP
ILIM
BST
DH
DL
LX
14
13
12
11
10
9
8
VOUT
1.5V
+5V
D1
C2
R1
2
+VIN
1
Q1
Q2
Figure 6
+
C1
3
Cout
D2
FBK
L1
Fs
Fs
-
+
200000
C3
Fs
1
www.semtech.com
R2
R3
D
SC488
2
C4
10pF
0.5V - 5.5V