lt1976 Linear Technology Corporation, lt1976 Datasheet - Page 14

• Manufacturer Part Number: lt1976
• Description: High Voltage 1.5a, 200khz Step-down Switching Regulator With 100?a Quiescent Current
• Manufacturer: Linear Technology Corporation

LT1976/LT1976B
APPLICATIO S I FOR ATIO
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E CASE SIZE
AVX TPS
D CASE SIZE
AVX TPS
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
triangular with a typical value of 200mA
to calculate this is:
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). Low ESR reduces output
ripple voltage but also removes a useful zero in the loop
frequency response, common to tantalum capacitors. To
compensate for this a resistor R
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
14
Output capacitor ripple current (RMS)
I
RIPPLE RMS
(
C
compensation capacitor C
)
=
0 29
ESR MAX (Ω)
U
.
0.1 to 0.3
0.1 to 0.3
0.2
(
V
( )( )( )
OUT
U
L f V
)(
V
IN
C
IN
can be placed in series
–
W
RIPPLE CURRENT (A)
V
OUT
C
RMS
(Figure 10). Care
0.7 to 1.1
0.7 to 1.1
)
0.5
. The formula
=
I
U
P-P
12
filter capacitor C
with a small feedforward capacitor C
control possible ripple at the V
stabilized using a 47μF ceramic output capacitor and V
component values of C
and C
OUTPUT RIPPLE VOLTAGE
Figure 3 shows a typical output ripple voltage waveform
for the LT1976. Ripple voltage is determined by the
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 ripple current slew rate
is also relevant and can be calculated from:
Peak-to-peak output ripple voltage is the sum of a triwave
created by peak-to-peak ripple current times ESR and a
square wave created by parasitic inductance (ESL) and
ripple current slew rate. Capacitive reactance is assumed
to be small compared to ESR or ESL.
47μF TANTALUM
47μF CERAMIC
I
V
dt
di
P-P
ESR 100mΩ
RIPPLE
20mV/DIV
20mV/DIV
FB
=
=
5V/DIV
= 27pF.
V
V
V
V
Figure 3. LT1976 Ripple Voltage Waveform
L
OUT
OUT
V
IN
SW
OUT
=
V
V
I
L = 33μH
LOAD
( )( )( )
IN
OUT
( )(
V
I
= 12V
(
P-P
IN
= 3.3V
V
= 1A
F
IN
in parallel with the R
L f
–
ESR
V
C
OUT
= 0.047μF, R
)
+
)
(
1μs/DIV
ESL
C
)
pin. The LT1976 can be
dt
di
C
C
FB
= 12.5k, C
/C
, is suggested to
C
network, along
1976 F03
F
= 100pF
1976bfg
C