13
LTC1736
APPLICATIO S I FOR ATIO
W
U
U
U
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:
C
L
I
V V
OUT
OUT
>
( )
( )
∆
∆
2
2
where
∆
I is the change in load current.
Larger diodes can result in additional transition losses due
to their larger junction capacitance. 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
I
V
V
V
V
RMS
O MAX
OUT
IN
IN
OUT
≅
(
)
/
– 1
1 2
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly
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 deter-
mined by:
∆
∆
V
I ESR
fC
OUT
L
OUT
≈
+
1
8
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)
the output ripple will be less than
50mV at max V
IN
assuming:
