LTC3718EG Linear Technology, LTC3718EG Datasheet - Page 11

• Manufacturer Part Number: LTC3718EG
• Description: IC DC/DC CONTRLR DDR/QDR 24-SSOP
• Manufacturer: Linear Technology

Specifications of LTC3718EG

• Applications: Controller, DDR, QDR
• Voltage - Input: 1.5 ~ 36 V
• Number Of Outputs: 1
• Voltage - Output: 0.75 ~ 18 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
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 LTC3718 applications is deter-
mined implicitly by the one-shot timer that controls the
on-time t
by the current into the I
pin according to:
P
P
D
D
TOP
BOT
BOT
TOP
= D
= D
ON
+ k V
V
V
TOP
BOT
of the top MOSFET switch. The on-time is set
2.0
1.0
0.5
1.5
V
OUT
IN
0
IN
– 50
Figure 2. R
IN
–
V
I
I
OUT(MAX)
OUT(MAX)
IN
2
V
OUT
I
U
OUT(MAX)
JUNCTION TEMPERATURE ( C)
0
2
ON
DS(ON)
R losses and the top MOSFET
2
2
U
pin and the voltage at the V
T(TOP)
T(BOT)
C
50
vs Temperature
RSS
f
R
R
W
DS(ON)(MAX)
DS(ON)(MAX)
100
3718 F02
150
U
–1
can
ON
Tying a resistor R
time inversely proportional to V
converter, this results in approximately constant fre-
quency 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 0.7V, 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.
Because the voltage at the I
current into this pin is not exactly inversely proportional to
V
To account for the 0.7V drop on the I
equation can be used to calculate frequency:
To correct for this error, an additional resistor R
connected from the I
further stabilize the frequency.
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
load current increases. By lengthening the on-time slightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the I
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the I
shown in Figure 3a. Place capacitance on the V
IN
R
t
f
f
, especially in applications with lower input voltages.
ON
ON
V
V
2
VON ON
VON
ON
V
V
I
IN
ION
TH
VON
pin to V
0 7 .
R
V
5
•
OUT
V
V
pin to the V
V
0 7
IN
(
10
(
.
R
10
•
ON
ON
pF
V V
R
OUT
pF
ON
from V
)
ON
•
)
. The V
(
10
OUT
pin to the 5V INTV
Hz
pF
ON
IN
)
ON
to the I
ON
pin and V
pin has internal clamps
pin is about 0.7V, the
TH
IN
ON
pin to the V
. For a step-down
ON
pin, the following
pin yields an on-
OUT
LTC3718
CC
. The values
supply will
ON
ON
11
pin as
pin to
3718fa
ON2