LTC3770EUH#TR Linear Technology, LTC3770EUH#TR Datasheet - Page 12

• Manufacturer Part Number: LTC3770EUH#TR
• Manufacturer: Linear Technology

Specifications of LTC3770EUH#TR

• Lead Free Status / RoHS Status: Not Compliant

APPLICATIONS INFORMATION
LTC3770
The ρ
accounting for the signifi cant variation in on-resistance
with temperature, typically about 0.4%/°C as shown in
Figure 1. For a maximum junction temperature of 100°C,
using a value ρ
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and the
load current. When the LTC3770 is operating in continuous
mode, the duty cycles for the MOSFETs are:
The resulting power dissipation in the MOSFETs at
maximum output current are:
Both MOSFETs have I
an additional term for transition losses, which are largest at
high input voltages. The constant k = 1.7A
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
effi ciency and component size. Low frequency operation
improves effi ciency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3770 applications is
determined implicitly by the one-shot timer that controls
the on-time t
is set by the current out of the I
the V
12
P
P
D
D
t
TOP
ON
BOT
BOT
TOP
ON
T
=
= D
= D
term is a normalization factor (unity at 25°C)
pin according to:
=
=
V
I
+ k V
ION
V
VON
V
TOP
BOT
V
IN
OUT
IN
ON
–
V
(
T
IN
I
I
10
IN
OUT(MAX)
V
OUT(MAX)
of the top MOSFET switch. The on-time
= 1.3 is reasonable.
2
OUT
pF
I
OUT(MAX)
2
R losses and the top MOSFET includes
)
2
2
ρ
ρ
T(TOP)
T(BOT)
C
RSS
ON
f
R
R
pin and the voltage at
DS(ON)(MAX)
DS(ON)(MAX)
–1
can be used to
Tying a resistor R
on-time inversely proportional to 1/3 V
of the I
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
To hold frequency constant during output voltage changes,
tie the V
that limit its input to the one-shot timer. If the pin is tied
below 0.6V, the input to the one-shot is clamped at 0.6V.
Similarly, if the pin is tied above 4.8V, the input is clamped
at 4.8V. In high V
2a and 2b show how R
for several common output voltages.
I
f
ION
=
Figure 2b. Switching Frequency vs R
Figure 2a. Switching Frequency vs R
V
=
ON
VON
ON
3
pin is:
V
R
pin to V
IN
•
ON
1000
1000
3
V
100
100
OUT
R
OUT
ON
ON
V
100
10
OUT
applications, tie V
OUT
(
V
10
OUT
to SGND from the I
= 1.5V
= 3.3V
pF
. The V
ON
)
relates to switching frequency
R
R
[
ON
ON
H
V
100
OUT
(kΩ)
(kΩ)
Z
ON
= 3.3V
V
]
OUT
pin has internal clamps
= 12V
V
OUT
V
ON
ON
OUT
= 2.5V
IN
ON
3770 F02b
3770 F02a
to INTV
= 5V
(V
. The current out
1000
ON
1000
(V
ON
ON
= INTV
pin yields an
= 0V)
CC
. Figures
CC
)
3770fc