Datasheet

11
LTC3778
3778f
APPLICATIO S I FOR ATIO
WUUU
The ρ
T
term is a normalization factor (unity at 25°C)
accounting for the significant variation in on-resistance
with temperature, typically about 0.4%/°C as shown in
Figure 2. For a maximum junction temperature of 100°C,
using a value ρ
T
= 1.3 is reasonable.
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 LTC3778 applications is deter-
mined implicitly by the one-shot timer that controls the
on-time t
ON
of the top MOSFET switch. The on-time is set
by the current into the I
ON
pin and the voltage at the V
ON
pin according to:
t
V
I
pF
ON
VON
ION
= ()10
Tying a resistor R
ON
from V
IN
to the I
ON
pin yields an on-
time inversely proportional to V
IN
. For a step-down con-
verter, this results in approximately constant frequency
operation as the input supply varies:
f
V
VR pF
Hz
OUT
VON ON
=
()()
[]
10
To hold frequency constant during output voltage changes,
tie the V
ON
pin to V
OUT
. The V
ON
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.
Similarly, if the pin is tied above 2.4V, the input is clamped
at 2.4V. If output is above 2.4V, use a resistive divider from
V
OUT
to V
ON
pin.
Because the voltage at the I
ON
pin is about 0.7V, the
current into this pin is not exactly inversely proportional to
V
IN
, especially in applications with lower input voltages.
To correct for this error, an additional resistor R
ON2
connected from the I
ON
pin to the 5V INTV
CC
supply will
further stabilize the frequency.
R
V
V
R
ON ON2
5
07
=
.
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
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON-RESISTANCE
1.0
1.5
150
3778 F02
0.5
0
0
50
100
2.0
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and
the load current. When the LTC3778 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:
P
TOP
= D
TOP
I
OUT(MAX)
2
ρ
T(TOP)
R
DS(ON)(MAX)
+ k V
IN
2
I
OUT(MAX)
C
RSS
f
P
BOT
= D
BOT
I
OUT(MAX)
2
ρ
T(BOT)
R
DS(ON)(MAX)
Both MOSFETs have I
2
R 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.
Figure 2. R
DS(ON)
vs. Temperature