ltc3770 Linear Technology Corporation, ltc3770 Datasheet - Page 12

• Manufacturer Part Number: ltc3770
• Description: Fast No Rsense Step-down Synchronous Controller With Margining, Tracking And Pll
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3770
The ρ
accounting for the significant 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 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 LTC3770 applications is deter-
mined implicitly by the one-shot timer that controls the
on-time t
by the current out of the I
pin according to:
12
P
P
D
D
t
ON
TOP
BOT
BOT
TOP
T
=
= D
= D
term is a normalization factor (unity at 25°C)
=
=
ON
V
+ k V
I
ION
VON
V
V
TOP
BOT
of the top MOSFET switch. The on-time is set
V
IN
OUT
IN
–
V
IN
T
(
I
I
10
IN
OUT(MAX)
OUT(MAX)
= 1.3 is reasonable.
V
2
OUT
I
pF
U
OUT(MAX)
)
2
R losses and the top MOSFET
ON
2
2
U
ρ
ρ
pin and the voltage at the V
T(TOP)
T(BOT)
C
RSS
f
R
R
W
DS(ON)(MAX)
DS(ON)(MAX)
U
–1
can
ON
Tying a resistor R
time inversely proportional to 1/3 V
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
Figures 2a and 2b show how R
frequency for several common output voltages.
I
f
ION
ON
=
V
=
pin is:
VON
ON
3
V
R
pin to V
Figure 2a. Switching Frequency vs R
Figure 2b. Switching Frequency vs R
IN
•
ON
3
V
1000
1000
100
100
OUT
R
ON
100
10
ON
OUT
OUT
to SGND from the I
(
V
10
OUT
V
OUT
(V
= 1.5V
. The V
pF
applications, tie V
= 3.3V
(V
ON
)
ON
= INTV
[
R
R
H
= 0V)
ON
ON
V
100
Z
ON
OUT
(kΩ)
(kΩ)
]
V
= 3.3V
CC
OUT
ON
pin has internal clamps
)
= 12V
IN
relates to switching
V
. The current out of
OUT
V
ON
OUT
= 2.5V
3770 F02b
3770 F02a
= 5V
pin yields an on-
1000
1000
ON
ON
ON
to INTV
3770fb
CC
.