LTC3714 Linear, LTC3714 Datasheet - Page 20

• Manufacturer Part Number: LTC3714
• Description: Wide Operating Range / Step-Down Controller with Internal Op Amp
• Manufacturer: Linear

APPLICATIO S I FOR ATIO
LTC3714
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Although all dissipative
elements in the circuit produce losses, four main sources
account for most of the losses in LTC3714 circuits:
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 15mW up to 1.5W 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. This loss can be reduced by supply-
ing INTV
efficiency source, such as an output derived boost net-
work or alternate supply if available.
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.
20
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
CC
current. This is the sum of the MOSFET driver
current through the EXTV
U
DS(ON)
OUT
U
–1
= 0.01 and R
) V
ESR loss, Schottky diode D1
IN
2
W
I
OUT
CC
C
pin from a high
DS(ON)
RSS
L
= 0.005 , the
2
f
U
R loss and
, then the
2
R
When making any adjustments to improve efficiency, the
final arbiter is the total input current for the regulator at
your operating point. If you make a change and the input
current decreases, then you improved the efficiency. 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
discharge C
the regulator to return V
During this recovery time, V
overshoot or ringing that would indicate a stability prob-
lem. The I
will provide adequate compensation for most applica-
tions. For a detailed explanation of switching control loop
theory see Linear Technology Application Note 76.
Design Example
As a design example, take a supply with the following
specifications: V
V
calculate the timing resistor with V
and choose the inductor for about 40% ripple current at
the maximum V
Choosing a standard value of 0.68 H results in a maxi-
mum ripple current of:
OUT
R
L
ON
I
L
= 1.15V 100mV, I
300
TH
300
LOAD
300
OUT
kHz
pin external components shown in Figure 8
1 15
kHz
kHz
generating a feedback error signal used by
.
IN
OUT
1 15
(ESR), where ESR is the effective series
.
:
0 4 15
1
V
.
IN
.
0 68
OUT
10
V
.
= 7V to 24V (15V nominal),
I
pF
LOAD
OUT(MAX)
immediately shifts by an amount
A
OUT
H
1
330
also begins to charge or
OUT
1
to its steady-state value.
–
1 15
= 15A, f = 300kHz. First,
k
24
1 15
can be monitored for
.
ON
24
.
V
V
V
= V
V
OUT
0 6
5 4
.
:
.
A
H
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