LT1766 Linear Technology, LT1766 Datasheet - Page 13

• Manufacturer Part Number: LT1766
• Description: 5.5V to 60V 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
its normal value. Below this point the oscillator frequency
decreases roughly linearly down to a limit of about 40kHz.
This lower oscillator frequency during short-circuit condi-
tions can then maintain control with the effective mini-
mum on time.
It is recommended that for [V
a soft-start circuit should be used to control the output
capacitor charge rate during start-up or during recovery
from an output short circuit, thereby adding additional
control over peak inductor current. See Buck Converter
with Adjustable Soft-Start later in this data sheet.
OUTPUT CAPACITOR
The output capacitor is normally chosen by its effective
series resistance (ESR), because this is what determines
output ripple voltage. To get low ESR takes volume , so
physically smaller capacitors have high ESR. The ESR
range for typical LT1766 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 2 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
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
U
ESR (Max,
U
0.1 to 0.3
0.1 to 0.3
0.2 (typ)
IN
/(V
OUT
)
W
+ V
Ripple Current (A)
F
)] ratios > 10,
0.7 to 1.1
0.7 to 1.1
0.5 (typ)
U
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
triangular with a typical value of 125mA
to calculate this is:
Output capacitor ripple current (RMS):
Ceramic Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available. They are generally chosen for their
good high frequency operation, small size and very low
ESR (effective series resistance). Their low ESR reduces
output ripple voltage but also removes a useful zero in the
loop frequency response, common to tantalum capaci-
tors. To compensate for this, a resistor R
in series with the V
must be taken however, since this resistor sets the high
frequency gain of the error amplifier, including the gain at
the switching frequency. If the gain of the error amplifier
is high enough at the switching frequency, output ripple
voltage (although smaller for a ceramic output capacitor)
may still affect the proper operation of the regulator. A
filter capacitor C
suggested to control possible ripple at the V
Ceramic” solution is possible for the LT1766 by choosing
the correct compensation components for the given
application.
Example: For V
LT1766 can be stabilized, provide good transient re-
sponse and maintain very low output ripple voltage using
the following component values: (refer to the first page of
this data sheet for component references) C3 = 2.2 F,
R
Application Note 19 for further detail on techniques for
proper loop compensation.
C
I
= 4.7k, C
RIPPLE RMS
C
= 15nF, C
IN
F
0 29
= 8V to 40V, V
in parallel with the R
.
C
compensation capacitor C
V
F
OUT
L f V
LT1766/LT1766-5
= 220pF and C1 = 47 F. See
V
IN
IN
OUT
V
OUT
= 3.3V at 1A, the
RMS
C
C
/C
can be placed
. The formula
C
C
pin. An “All
network is
13
C
. Care
1766fa