Datasheet

15
LTC1266
LTC1266-3.3/LTC1266-5
operation, the DC supply current represents the lone (and
unavoidable) loss component which continues to become
a higher percentage as output current is reduced. As
expected the I
2
R losses dominate at high load currents.
Other losses including C
IN
and C
OUT
ESR dissipative
losses, MOSFET switching losses, Schottky conduction
losses during deadtime and inductor core losses, gener-
ally account for less than 2% total additional loss.
Design Example
As a design example, assume V
IN
= 5V (nominal),
V
OUT
= 3.3V, I
MAX
= 5A and f = 200kHz; R
SENSE
, C
T
and L
can immediately be calculated:
R
SENSE
= 100mV/5 = 0.02Ω
t
OFF
= (1/200kHz) • [1 – (3.3/5)] = 1.7µs
C
T
= 1.7µs/(1.3 • 10
4
) = 130pF
L
MIN
= 5.1 • 10
5
• 0.02Ω • 130pF • 3.3V = 5µH
Assume that the MOSFET dissipations are to be limited to
P
T
= P
B
= 2W.
If T
A
= 40°C and the thermal resistance of each MOSFET
is 50°C/W, then the junction temperatures will be 140°C
and δ
T
= δ
B
= 0.60. The required R
DS(ON)
for each MOSFET
can now be calculated:
TS R
DS(ON)
=
5(2)
3.3(5)
2
(1.60)
= 0.076Ω
BS R
DS(ON)
=
5(2)
1.7(5)
2
(1.60)
= 0.147Ω
The topside FET requirement can be met by an N-channel
Si9410DY which has an R
DS(ON)
of about 0.04Ω at
V
GS
= 5V. The bottom-side FET requirement is exceeded
by an Si9410DY. Note that the most stringent requirement
for the bottom-side MOSFET is with V
OUT
= 0 (i.e., short
circuit). During a continuous short circuit, the worst-case
dissipation rises to:
P
B
= I
SC(AVG)
2
• R
DS(ON)
• (1 + δ
B
)
With the 0.02Ω sense resistor, I
SC(AVG)
≈ 6A will result,
increasing the 0.04Ω bottom-side FET dissipation to 2.3W.
C
IN
will require an RMS current rating of at least 2.5A at
temperature and C
OUT
will require an ESR of 0.02Ω for
optimum efficiency.
Now allow V
IN
to drop to its minimum value. The minimum
V
IN
can be calculated from the maximum duty cycle and
voltage drop across the topside FET,
V
MIN
=
D
MAX
V
OUT
+ I
LOAD
• (R
DS(ON)
+ R
L
+ R
SENSE
)
= 4.0V
At this lower input voltage, the operating frequency de-
creases and the topside FET will be conducting most of the
time, causing the power dissipation to increase.
At dropout,
f
MIN
=
1
t
ON (MAX)
+ t
OFF
= 16kHz
P
T
= I
2
LOAD
• R
DS(ON)
• (1 + δ
T
) • D
MAX
This last step is necessary to assure that the power
dissipation and junction temperature of the topside FET
are not exceeded.
These last calculations assume that Power V
IN
is high
enough to keep the topside FET fully turned on at dropout,
as would be the case with the Figure 11circuit. If this isn’t
true (as with the Figure 1 circuit) the R
DS(ON)
will increase
which in turn increases V
MIN
and P
T
.
Adjustable Applications
When an output voltage other than 3.3V or 5V is required,
the LTC1266 adjustable version is used with an external
resistive divider from V
OUT
to V
FB
, Pin 10. The regulated
voltage is determined by:
V
OUT
= 1.265
) )
1 +
R2
R1
To prevent stray pickup a 100pF capacitor is suggested
across R1 located close to the LTC1266.
For Figure 1 applications with V
OUT
below 2V, or when
R
SENSE
is moved to ground, the current sense comparator
inputs operate near ground. When the current comparator
is operated at less than 2V common mode, the off-time
increases approximately 40%, requiring the use of a
smaller timing capacitor C
T
.
APPLICATIO S I FOR ATIO
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