LT1374C Linear Technology, LT1374C Datasheet - Page 12

• Manufacturer Part Number: LT1374C
• Description: 4.5A/ 500kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT1374
APPLICATIONS
Output Capacitor
The output capacitor is normally chosen by its Effective
Series Resistance (ESR), because this is what determines
output ripple voltage. At 500kHz, any polarized capacitor
is essentially resistive. To get low ESR takes volume , so
physically smaller capacitors have high ESR. The ESR
range for typical LT1374 applications is 0.05 to 0.2 . A
typical output capacitor is an AVX type TPS, 100 F at 10V,
with a guaranteed ESR less than 0.1 . This is a “D” size
surface mount solid tantalum capacitor. TPS capacitors
are specially constructed and tested for low ESR, so they
give the lowest ESR for a given volume. The value in
microfarads is not particularly critical, and values from
22 F to greater than 500 F work well, but you cannot
cheat mother nature on ESR. If you find a tiny 22 F solid
tantalum capacitor, it will have high ESR, and output ripple
voltage will be terrible. Table 3 shows some typical solid
tantalum surface mount capacitors.
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E Case Size
AVX TPS, Sprague 593D
AVX TAJ
D Case Size
AVX TPS, Sprague 593D
C Case Size
AVX TPS
Many engineers have heard that solid tantalum capacitors
are prone to failure if they undergo high surge currents.
This is historically true, and type TPS capacitors are
specially tested for surge capability, but surge ruggedness
is not a critical issue with the output capacitor. Solid
tantalum capacitors fail during very high turn-on surges,
which do not occur at the output of regulators. High
discharge surges, such as when the regulator output is
dead shorted, do not harm the capacitors.
Unlike the input capacitor, RMS ripple current in the
output capacitor is normally low enough that ripple cur-
rent rating is not an issue. The current waveform is
12
U
ESR (Max.,
INFORMATION
U
0.1 to 0.3
0.7 to 0.9
0.1 to 0.3
0.2 (typ)
W
)
Ripple Current (A)
0.7 to 1.1
0.7 to 1.1
0.5 (typ)
0.4
U
triangular with a typical value of 200mA
to calculate this is:
Output Capacitor Ripple Current (RMS):
Ceramic Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available in smaller case sizes. These are tempt-
ing for switching regulator use because of their very low
ESR. Unfortunately, the ESR is so low that it can cause
loop stability problems. Solid tantalum capacitor’s ESR
generates a loop “zero” at 5kHz to 50kHz that is instrumen-
tal in giving acceptable loop phase margin. Ceramic
capacitors remain capacitive to beyond 300kHz and usu-
ally resonate with their ESL before ESR becomes effective.
They are appropriate for input bypassing because of their
high ripple current ratings and tolerance of turn-on surges.
Linear Technology plans to issue a design note on the use
of ceramic capacitors in the near future.
OUTPUT RIPPLE VOLTAGE
Figure 3 shows a typical output ripple voltage waveform
for the LT1374. Ripple voltage is determined by the high
frequency impedance of the output capacitor, and ripple
current through the inductor. Peak-to-peak ripple current
through the inductor into the output capacitor is:
For high frequency switchers, the sum of ripple current
slew rates may also be relevant and can be calculated
from:
I
I
RIPPLE RMS
P
-P
dt
dI
V
V
L
OUT
IN
V
IN
V
0 29
IN
L f
V
V
OUT
OUT
L f V
V
IN
IN
V
OUT
RMS
. The formula