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LTR3800 Datasheet(PDF) 11 Page - Linear Technology

Part # LTR3800
Description  Wide Operating Range, No RSENSE Step-Down Controller
Download  24 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

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11
LTC1778/LTC1778-1
1778fb
APPLICATIO S I FOR ATIO
with temperature, typically about 0.4%/°C as shown in
Figure 2. For a maximum junction temperature of 100°C,
using a value ρT = 1.3 is reasonable.
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and
the load current. When the LTC1778 is operating in
continuous mode, the duty cycles for the MOSFETs are:
D
V
V
D
VV
V
TOP
OUT
IN
BOT
IN
OUT
IN
=
=
The resulting power dissipation in the MOSFETs at maxi-
mum output current are:
PTOP = DTOP IOUT(MAX)2 ρT(TOP) RDS(ON)(MAX)
+ k VIN2 IOUT(MAX) CRSS f
PBOT = DBOT IOUT(MAX)2 ρT(BOT) RDS(ON)(MAX)
Both MOSFETs have I2R losses and the top MOSFET
includes an additional term for transition losses, which are
largest at high input voltages. The constant k = 1.7A–1 can
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 LTC1778 applications is deter-
mined implicitly by the one-shot timer that controls the
on-time tON of the top MOSFET switch. The on-time is set
by the current into the ION pin and the voltage at the VON
pin (LTC1778-1) according to:
t
V
I
pF
ON
VON
ION
=
()
10
VON defaults to 0.7V in the LTC1778.
Tying a resistor RON from VIN to the ION pin yields an on-
time inversely proportional to VIN. For a step-down con-
verter, this results in approximately constant frequency
operation as the input supply varies:
f
V
VR
pF
H
OUT
VON
ON
Z
=
()
[]
10
To hold frequency constant during output voltage changes,
tie the VON pin to VOUT or to a resistive divider from VOUT
when VOUT > 2.4V. The VON pin has internal 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. Simi-
larly, if the pin is tied above 2.4V, the input is clamped at
2.4V. In high VOUT applications, tying VON to INTVCC so
that the comparator input is 2.4V results in a lower value
for RON. Figures 3a and 3b show how RON relates to
switching frequency for several common output voltages.
RON (kΩ)
100
100
1000
1000
10000
1778 F03a
VOUT = 3.3V
VOUT = 1.5V
VOUT = 2.5V
RON (kΩ)
100
100
1000
1000
10000
1778 F03b
VOUT = 3.3V
VOUT = 12V
VOUT = 5V
Figure 3a. Switching Frequency vs RON
for the LTC1778 and LTC1778-1 (VON = 0V)
Figure 3b. Switching Frequency vs RON
for the LTC1778-1 (VON = INTVCC)


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