10
LTC1159
LTC1159-3.3/LTC1159-5
C
IN
and C
OUT
Selection
In continuous mode, the source current of the P-channel
MOSFET is a square wave of duty cycle V
OUT
/V
IN
.
To prevent large voltage transients, a low ESR input
capacitor sized for the
maximum RMS current must be
used. The maximum RMS capacitor current is given by:
C
IN
Required I
RMS
≈
I
MAX
[V
OUT
(V
IN
–
V
OUT
)]
1/2
V
IN
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
MAX
/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on only 2000 hours
of life. This makes it advisable to further derate the
capacitor, or to choose a capacitor rated at a higher
temperature than required. Several capacitors may be
paralleled to meet size or height requirements in the
design. An additional 0.1µF ceramic capacitor may also be
required on V
IN
for high frequency decoupling.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). The ESR of C
OUT
must be less than
twice the value of R
SENSE
for proper operation of the
LTC1159:
C
OUT
Required ESR < 2R
SENSE
Optimum efficiency is obtained by making the ESR equal to
R
SENSE
. Manufacturers such as Nichicon, Chemicon, and
Sprague should be considered for high performance ca-
pacitors. The OS-CON semiconductor dielectric capacitor
available from Sanyo has the lowest ESR for its size at a
somewhat higher price. Once the ESR requirement for
C
OUT
has been met, the RMS current rating generally far
exceeds the I
RIPPLE(P-P)
requirement.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirements of the application. Alumi-
num electrolytic and dry tantalum capacitors are both
available in surface mount configurations. In the case of
tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice is
the AVX TPS series of surface mount tantalums, available
in case heights ranging from 2mm to 4mm. For example,
if 200µF/10V is called for in an application requiring 3mm
height, two AVX 100µF/10V (P/N TPSD107K010) could be
used. Consult the manufacturer for other specific recom-
mendations.
At low supply voltages, a minimum value of C
OUT
is
suggested to prevent an abnormal low frequency operating
mode (see Figure 4). When C
OUT
is too small, the output
ripple at low frequencies will be large enough to trip the
voltage comparator. This causes the Burst Mode operation
to be activated when the LTC1159 would normally be in
continuous operation. The effect is most pronounced with
low values of R
SENSE
and can be improved by operating at
higher frequencies with lower values of L. The output
remains in regulation at all times.
Figure 4. Minimum Suggested C
OUT
(V
IN
– V
OUT
) VOLTAGE (V)
0
C
OUT
(µF)
600
800
1000
4
LTC1159 • TPC04
400
200
0
1
2
3
5
L = 50µH
R
SENSE
= 0.02Ω
L = 25µH
R
SENSE
= 0.02Ω
L = 50µH
R
SENSE
= 0.05Ω
Load Transient Response
Switching regulators take several cycles to respond to a
step in DC (resistive) load current. When a load step
occurs, V
OUT
shifts by an amount equal to ∆I
LOAD
• ESR,
where ESR is the effective series resistance of C
OUT
. ∆I
LOAD
also begins to charge or discharge C
OUT
until the regulator
loop adapts to the current change and returns V
OUT
to its
steady-state value. During this recovery time V
OUT
can be
monitored for overshoot or ringing which would indicate a
stability problem. The I
TH
external components shown in
the Figure 1 circuit will provide adequate compensation for
most applications.
A second, more severe transient is caused by switching in
loads with large (>1µF) supply bypass capacitors. The
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