LTC3709 Linear Technology, LTC3709 Datasheet - Page 19

• Manufacturer Part Number: LTC3709
• Description: No RSENSE Synchronous DC/DC Controller
• Manufacturer: Linear Technology

www.DataSheet4U.com
APPLICATIO S I FOR ATIO
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 LTC3709 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 0.1% 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. Gate driver supply current. The driver current supplies
the gate charge Q
This current is typically much larger than the control
circuit current. In continuous mode operation:
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
I
R
Transition Loss
GATECHG
IN
DS ON DRV
2
(
loss. The input capacitor has the difficult job of
R losses. These arise from the resistances of the
)_
= f (Q
⎛
⎜
⎝
G
DRV
g(TOP)
required to switch the power MOSFETs.
U
≈
DS(ON)
( . ) •
CC
0 5
+ Q
−
U
1
V
g(BOT)
= 0.01Ω and R
V
GS TH
IN
(
2
•
)
)
I
W
OUT
+
V
GS TH
•
C
DS(ON)
1
(
L
RSS
= 0.005Ω, the
)
2
⎞
⎟
⎠
U
R loss and
• •
, then the
f
2
R
causing additional upstream losses in fuses or batteries.
Other losses, including C
tion loss during dead time and inductor core loss generally
account for less than 2% additional loss.
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-
lems. The I
will provide adequate compensation for most applica-
tions. 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
2.5V, I
timing resistor:
and choose the inductor for about 40% ripple current at
the maximum V
channel is 10A:
R
L
ON
=
OUT(MAX)
(
=
250
(
TH
LOAD
0 7
OUT
kHz
.
pin external components shown in Figure 9
V
2 5
generating a feedback error signal used by
)( )(
OUT
)(
(ESR), where ESR is the effective series
.
IN
= 20A, f = 250kHz. First, calculate the
IN
0 4 10
250
V
. Maximum output current for each
.
= 7V to 28V (15V nominal), V
2 5
. ∆I
OUT
.
kHz
V
LOAD
immediately shifts by an amount
OUT
A
)(
OUT
)
30
⎛
⎜
⎝
ESR loss, Schottky conduc-
1
also begins to charge or
OUT
pF
to its steady-state value.
−
)
2 5
28
can be monitored for
=
.
V
476
V
⎞
⎟ =
⎠
k
LTC3709
2 3
.
µ
H
19
OUT
3709f
=