LTC1735 Linear Technology, LTC1735 Datasheet - Page 13

• Manufacturer Part Number: LTC1735
• Description: High Efficiency Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
The Schottky diode D1 shown in Figure 1 conducts during the
dead-time between the conduction of the two power MOSFETs.
This prevents the body diode of the bottom MOSFET from
turning on and storing charge during the dead-time, which
could cost as much as 1% in efficiency. A 3A Schottky is
generally a good size for 10A to 12A regulators due to the
relatively small average current. Larger diodes can result in
additional transition losses due to their larger junction capaci-
tance. The diode may be omitted if the efficiency loss can be
tolerated.
C
In continuous mode, the source current of the top
N-channel MOSFET is a square wave of duty cycle V
V
capacitor sized for the maximum RMS current must be
used. The maximum RMS capacitor current is given by:
This formula has a maximum at V
I
monly used for design because even significant deviations do
not offer much relief. Note that capacitor manufacturers’
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 also be paralleled to meet
size or height requirements in the design. Always consult the
manufacturer if there is any question.
C
The selection of C
effective series resistance (ESR) to minimize voltage
ripple. The output ripple ( V
determined by:
Where f = operating frequency, C
tance and I
ripple is highest at maximum input voltage since I
RMS
IN
IN
OUT
I
. To prevent large voltage transients, a low ESR input
RMS
Selection
V
= I
Selection
OUT
O(MAX)
I
O MAX
(
L
/2. This simple worst case condition is com-
= ripple current in the inductor. The output
I ESR
L
)
V
U
OUT
V
OUT
IN
is primarily determined by the
8
fC
U
V
V
OUT
1
OUT
IN
OUT
– 1
) in continuous mode is
OUT
W
1 2
IN
/
= output capaci-
= 2V
OUT
U
, where
OUT
L
/
increases with input voltage. Typically, once the ESR
requirement for C
rating generally far exceeds the I
With I
to ESR the output ripple will be less than 50mV at max V
assuming:
The first condition relates to the ripple current into the ESR
of the output capacitance while the second term guaran-
tees that the output capacitance does not significantly
discharge during the operating frequency period due to
ripple current. The choice of using smaller output capaci-
tance increases the ripple voltage due to the discharging
term but can be compensated for by using capacitors of
very low ESR to maintain the ripple voltage at or below
50mV. The I
nents can be optimized to provide stable, high perfor-
mance transient response regardless of the output capaci-
tors selected.
The selection of output capacitors for CPU or other appli-
cations with large load current transients is primarily
determined by the voltage tolerance specifications of the
load. The resistive component of the capacitor, ESR,
multiplied by the load current change plus any output
voltage ripple must be within the voltage tolerance of the
load (CPU).
The required ESR due to a load current step is:
where I is the change in current from full load to zero load
(or minimum load) and V is the allowed voltage deviation
(not including any droop due to finite capacitance).
The amount of capacitance needed is determined by the
maximum energy stored in the inductor. The capacitance
must be sufficient to absorb the change in inductor current
when a high current to low current transition occurs. The
opposite load current transition is generally determined by
the control loop OPTI-LOOP components, so make sure
not to over compensate and slow down the response. The
minimum capacitance to assure the inductors’ energy is
adequately absorbed is:
C
C
R
OUT
OUT
ESR
L
required ESR < 2.2 R
> 1/(8fR
< V/ I
= 0.3I
TH
OUT(MAX)
pin OPTI-LOOP compensation compo-
SENSE
OUT
)
has been met, the RMS current
and allowing 2/3 of the ripple due
SENSE
RIPPLE(P–P)
LTC1735
requirement.
13
IN