LTC3718EG#PBF Linear Technology, LTC3718EG#PBF Datasheet - Page 16

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

Specifications of LTC3718EG#PBF

• 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: Lead free / RoHS Compliant

APPLICATIO S I FOR ATIO
LTC3718
1. DC I
MOSFETs, inductor and PC board traces and cause the
efficiency to drop at high output currents. In continuous
mode the average output current flows through L, but is
chopped between the top and bottom MOSFETs. If the two
MOSFETs have approximately the same R
resistance of one MOSFET can simply be summed with the
resistances of L and the board traces to obtain the DC I
loss. For example, if R
loss will range from 1% up to 10% as the output current
varies from 1A to 10A for a 1.5V output.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated region
during switch node transitions. It depends upon the input
voltage, load current, driver strength and MOSFET capaci-
tance, among other factors. The loss is significant at input
voltages above 20V and can be estimated from:
3. INTV
and control currents.
4. C
filtering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I
sufficient capacitance to prevent the RMS current from
causing additional upstream losses in fuses or batteries.
Other losses, including C
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss.
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V
equal to I
resistance of C
16
Transition Loss (1.7A
IN
loss. The input capacitor has the difficult job of
2
CC
R losses. These arise from the resistances of the
current. This is the sum of the MOSFET driver
LOAD
OUT
(ESR), where ESR is the effective series
U
. I
OUT
DS(ON)
LOAD
immediately shifts by an amount
OUT
U
–1
= 0.01 and R
) V
ESR loss, Schottky diode D1
also begins to charge or
IN
2
W
I
OUT
C
DS(ON)
RSS
L
= 0.005 , the
2
f
U
R loss and
, then the
2
R
discharge C
by the regulator to return V
During this recovery time, V
overshoot or ringing that would indicate a stability
problem. The I
Figure 1 will provide adequate compensation for most
applications. For a detailed explanation of switching
control loop theory see Application Note 76.
Design Example
As a design example, take a supply with the following
specifications: V
I
resistor with V
Next, use a standard value of 237k and choose the inductor
for about 40% ripple current at the maximum V
Selecting a standard value of 1 H results in a maximum
ripple current of:
Next, choose the synchronous MOSFET switch. Choosing
an IRF7811A (R
50 C/W) yields a nominal sense voltage of:
Tying V
for a nominal value of 100mV with current limit occurring
at 133mV. To check if the current limit is acceptable,
assume a junction temperature of about 10 C above a
50 C ambient with
OUT(MAX)
V
R
I
L
LIMIT
SNS(NOM)
ON
I
L
(
RNG
300
(
= 6A, f = 300kHz. First, calculate the timing
( . )(
300
2 5
( . )( .
kHz
to 1V will set the current sense voltage range
OUT
1 15 0 013
1 25
= (6A)(1.3)(0.013 ) = 101.4mV
1 25
V
kHz
ON
.
2 5
.
133
)( . )( )
TH
generating a feedback error signal used
.
DS(ON)
300
0 4 6
IN
= V
V
)(
V
V
pin external components shown in
1
mV
60 C
= 2.5V, V
kHz
OUT
H
0 7
A
)
.
= 0.013 , C
:
)(
= 1.15:
V
1
10
)
1
–
OUT
–
pF
1 25
OUT
2 5
1
2
1 25
.
2 5
.
to its steady-state value.
OUT
)
( . )
.
2 1
.
V
V
can be monitored for
V
240
V
A
= 1.25V
RSS
k
2 1
9 9
0 87
.
.
= 60pF,
.
A
A
H
IN
100mV,
:
JA
3718fa
=