ltc3711 Linear Technology Corporation, ltc3711 Datasheet - Page 11

• Manufacturer Part Number: ltc3711
• Description: 5-bit Adjustable, Wide Operating Range, No Rsense? Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
The resulting power dissipation in the MOSFETs at maxi-
mum output current are:
Both MOSFETs have I
includes an additional term for transition losses, which are
largest at high input voltages. The constant k = 1.7A
be used to estimate the amount of transition loss. The
bottom MOSFET losses are greatest when the bottom duty
cycle is near 100%, during a short-circuit or at high input
voltage.
Operating Frequency
The choice of operating frequency is a tradeoff between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3711 applications is deter-
mined implicitly by the one-shot timer that controls the
on-time t
by the current into the I
pin according to:
P
P
t
ON
TOP
BOT
= D
= D
ON
V
+ k V
I
2.0
1.0
0.5
ION
1.5
VON
TOP
BOT
0
of the top MOSFET switch. The on-time is set
– 50
Figure 2. R
IN
(
I
I
10
OUT(MAX)
OUT(MAX)
2
JUNCTION TEMPERATURE ( C)
I
pF
OUT(MAX)
U
0
)
DS(ON)
2
ON
R losses and the top MOSFET
2
2
U
pin and the voltage at the V
50
T(TOP)
T(BOT)
C
vs. Temperature
RSS
f
R
R
W
100
DS(ON)(MAX)
DS(ON)(MAX)
3711 F02
150
U
–1
can
ON
Tying a resistor R
time inversely proportional to V
verter, this results in approximately constant frequency
operation as the input supply varies:
To hold frequency constant during output voltage changes,
tie the V
varies with R
clamps that limit its input to the one-shot timer. If the pin
is tied below 0.7V, the input to the one-shot is clamped at
0.7V. Similarly, if the pin is tied above 2.4V, the input is
clamped at 2.4V.
Because the voltage at the I
current into this pin is not exactly inversely proportional to
V
To correct for this error, an additional resistor R
connected from the I
further stabilize the frequency.
Changes in the load current magnitude will also cause
frequency shift. Parasitic resistance in the MOSFET
switches and inductor reduce the effective voltage across
the inductance, resulting in increased duty cycle as the
load current increases. By lengthening the on-time slightly
IN
Figure 3. Switching Frequency vs R
R
f
, especially in applications with lower input voltages.
ON
V
2
VON ON
ON
0 7 .
1000
pin to V
5
R
V
10
V
ON
OUT
100
V
R
(
in this case. The V
10
ON
ON
OUT
pF
from V
ON
)
. Figure 3 shows how frequency
pin to the 5V INTV
R
IN
Hz
ON
ON
to the I
(k )
IN
ON
pin is about 0.7V, the
. For a step-down con-
with V
ON
ON
pin yields an on-
pin has internal
ON
LTC3711
3711 F03
CC
1000
Tied to V
supply will
11
OUT
ON2
3711f