Datasheet

LT1977
12
1977fa
APPLICATIO S I FOR ATIO
WUUU
triangular with a typical value of 200mA
RMS
. The formula
to calculate this is:
Output capacitor ripple current (RMS)
I
VVV
LfV
RIPPLE RMS
OUT IN OUT
IN
()
.–
=
()( )
()()( )
=
029
12
I
P-P
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
C
can be placed in series
with the V
C
compensation capacitor C
C
(Figure 10). Care
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
F
in parallel with the R
C
/C
C
network, along
with a small feedforward capacitor C
FB
, is suggested to
control possible ripple at the V
C
pin. The LT1977 can be
stabilized using a 100µF ceramic output capacitor and V
C
component values of C
C
= 1500pF, R
C
= 10k, C
F
= 330pF
and C
FB
= 10pF.
OUTPUT RIPPLE VOLTAGE
Figure 3 shows a typical output ripple voltage waveform
for the LT1977. 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:
I
VVV
VLf
OUT IN OUT
IN
P-P
=
()
()()()
–
For high frequency switchers the ripple current slew rate
is also relevant and can be calculated from:
di
dt
V
L
IN
=
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.
V I ESR ESL
di
dt
RIPPLE
=
()( )
+
()
P-P
Example: with V
IN
= 12V, V
OUT
= 3.3V, L = 15µH, ESR =
0.08Ω, ESL = 10nH:
I
P-P
=
()( )
()
−
()()
=
==
33 12 33
12 15 6 500 3
0 319
12
15 6
08 6
.–.
.
–
.
ee
A
di
dt e
e
V
RIPPLE
= (0.319A)(0.08) + (10e – 9)(0.8e6)
= 0.026 + 0.008 = 34mV
P-P
MAXIMUM OUTPUT LOAD CURRENT
Maximum load current for a buck converter is limited by
the maximum switch current rating (I
PK
). The minimum
specified current rating for the LT1977 is 1.5A. Unlike
most current mode converters, the LT1977 maximum
switch current limit does not fall off at high duty cycles.
Most current mode converters suffer a drop off of peak
switch current for duty cycles above 50%. This is due to
the effects of slope compensation required to prevent
subharmonic oscillations in current mode converters.
(For detailed analysis, see Application Note 19.)
Figure 3. LT1977 Ripple Voltage Waveform
V
OUT
10mV/DIV
100µF
75mΩ
TANTALUM
V
OUT
10mV/DIV
100µF
CERAMIC
V
SW
10V/DIV
V
IN
= 12V
V
OUT
= 3.3V
I
L
= 1A
500ns/DIV
1977 F03