LTC1778EGN Linear Technology, LTC1778EGN Datasheet - Page 11

• Manufacturer Part Number: LTC1778EGN
• Description: IC CONTROLLR STP-DWN SYNC 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1778EGN

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Current - Output: 2A
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -
• Power - Output: -
• Frequency - Switching: -

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 ρ
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:
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 LTC1778 applications is deter-
mined implicitly by the one-shot timer that controls the
on-time t
by the current into the I
pin (LTC1778-1) according to:
V
ON
P
P
D
D
t
ON
TOP
BOT
BOT
TOP
defaults to 0.7V in the LTC1778.
=
= D
= D
=
=
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
T
IN
(
I
I
10
IN
OUT(MAX)
OUT(MAX)
= 1.3 is reasonable.
V
2
OUT
I
pF
U
OUT(MAX)
)
2
ON
R losses and the top MOSFET
2
2
U
pin and the voltage at the V
ρ
ρ
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 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
when V
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 V
that the comparator input is 2.4V results in a lower value
for R
switching frequency for several common output voltages.
f
=
ON
V
OUT
. Figures 3a and 3b show how R
ON
Figure 3a. Switching Frequency vs R
for the LTC1778 and LTC1778-1 (V
VON
1000
1000
100
100
Figure 3b. Switching Frequency vs R
for the LTC1778-1 (V
pin to V
> 2.4V. The V
100
100
V
R
OUT
ON
OUT
V
OUT
(
V
ON
10
OUT
OUT
= 3.3V
applications, tying V
LTC1778/LTC1778-1
= 1.5V
pF
from V
or to a resistive divider from V
)
[
R
R
H
ON
ON
ON
1000
1000
Z
IN
(kΩ)
(kΩ)
]
pin has internal clamps that
ON
V
V
to the I
OUT
OUT
V
= INTV
IN
V
OUT
= 3.3V
OUT
= 12V
. For a step-down con-
= 5V
= 2.5V
ON
CC
ON
)
1778 F03a
1778 F03b
ON
pin yields an on-
ON
= 0V)
10000
10000
ON
ON
to INTV
relates to
11
CC
1778fb
OUT
so