LTC1735 Datasheet, PDF (13/32 Pages) Linear Technology – High Efficiency Synchronous Step-Down Switching Regulator

LTC1735
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 Selection
In continuous mode, the source current of the top
N-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:
I_RMS
≅ I_O(MAX)
V_OUT
V_IN
 _VIN
 V_OUT
^{1/ 2}
– 1_
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS␣ =␣ I_O(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 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_OUT Selection
The selection of C_OUT is primarily determined by the
effective series resistance (ESR) to minimize voltage
ripple. The output ripple (∆V_OUT) in continuous mode is
determined by:
∆V_OUT
≈

∆I_{L }ESR
+
1
8fC_OUT


Where f = operating frequency, C_OUT = output capaci-
tance and ∆I_L = ripple current in the inductor. The output
ripple is highest at maximum input voltage since ∆I_L
increases with input voltage. Typically, once the ESR
requirement for C_OUT has been met, the RMS current
rating generally far exceeds the I_{RIPPLE(P–P)} requirement.
With ∆I_L = 0.3I_OUT(MAX) and allowing 2/3 of the ripple due
to ESR the output ripple will be less than 50mV at max V_IN
assuming:
C_OUT required ESR < 2.2 R_SENSE
C_OUT > 1/(8fR_SENSE)
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_TH pin OPTI-LOOP compensation compo-
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:
R_ESR < ∆V/∆I
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:
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