LTC1159 Linear Technology, LTC1159 Datasheet - Page 9

• Manufacturer Part Number: LTC1159
• Description: High Efficiency Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
where is the temperature dependency of R
is a constant inversely related to the gate drive current.
Both MOSFETs have I
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
high current efficiency generally improves with larger
MOSFETs, while for V
increase to the point that the use of a higher R
device with lower C
ciency. The N-channel MOSFET losses are the greatest at
high input voltage or during a short circuit when the N-
channel duty cycle is nearly 100%.
The term (1 + ) is generally given for a MOSFET in the form
of a normalized R
voltage MOSFETs. C
electrical characteristics. The constant k = 5 can be used for
the LTC1159 to estimate the relative contributions of the
two terms in the P-channel dissipation equation.
The Schottky diode D1 shown in Figure 1 only conducts
during the dead time between the conduction of the two
power MOSFETs. D1 prevents the body diode of the
N-channel MOSFET from turning on and storing charge
during the dead time, which could cost as much as 1% in
efficiency (although there are no other harmful effects if
D1 is omitted). Therefore, D1 should be selected for a
forward voltage of less than 0.6V when conducting I
C
In continuous mode, the source current of the P-channel
MOSFET is a square wave of duty cycle V
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:
IN
= 0.007/ C can be used as an approximation for low
P-Ch P
N-Ch P
C
and C
IN
Required I
OUT
D
D
=
=
Selection
V
k(V
V
V
IN
OUT
IN
RMS
IN
V
– V
RSS
U U
IN
DS(ON)
)
IN
2
RSS
(I
OUT
MAX
(I
> 20V the transition losses rapidly
is usually specified in the MOSFET
2
MAX
R losses while the P-channel
I
actually provides higher effi-
MAX
(I
)
vs Temperature curve, but
MAX
2
) (C
(1 +
[V
)
RSS
2
OUT
(1 +
W
P
) (f)
(V
) R
V
IN
IN
DS(ON)
N
– V
) R
DS(ON)
IN
OUT
DS(ON)
< 20V the
U
OUT
+
)]
DS(ON)
and k
1/2
/V
MAX
IN
.
.
This formula has a maximum at V
I
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 0.1 F ceramic capacitor may also be
required on V
The selection of C
series resistance (ESR). The ESR of C
twice the value of R
LTC1159:
Optimum efficiency is obtained by making the ESR equal to
R
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
exceeds the I
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 200 F/10V is called for in an application requiring 3mm
height, two AVX 100 F/10V (P/N TPSD107K010) could be
used. Consult the manufacturer for other specific recom-
mendations.
At low supply voltages, a minimum value of C
suggested to prevent an abnormal low frequency oper-
ating mode (see Figure 4). When C
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
LTC1159/LTC1159-3.3/LTC1159-5
RMS
OUT
SENSE
C
OUT
= I
has been met, the RMS current rating generally far
. Manufacturers such as Nichicon, Chemicon, and
MAX
Required ESR < 2R
/2. This simple worst case condition is com-
RIPPLE(P-P)
IN
for high frequency decoupling.
OUT
SENSE
is driven by the required effective
requirement.
SENSE
for proper operation of the
OUT
OUT
IN
must be less than
= 2V
is too small, the
OUT
, where
OUT
9
is