LTC3708 LINER [Linear Technology], LTC3708 Datasheet - Page 22

• Manufacturer Part Number: LTC3708
• Description: Fast 2-Phase, No RSENSE Buck Controller with Output Tracking
• Manufacturer: LINER [Linear Technology]

LTC3708
APPLICATIONS INFORMATION
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 signifi cant at input
voltages above 20V and can be estimated from:
3. DRV
driver and control currents. 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
fi ltering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I
suffi cient capacitance to prevent the RMS current from
causing additional upstream losses in fuses or batteries.
The LTC3708 2-phase architecture typically halves this C
loss over the single phase solutions.
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 effi ciency, the
fi nal 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 improve the effi ciency. If there
is no change in input current, then there is no change in
effi ciency.
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
22
I
( . ) •
R
Transition Loss
GATECHG
0 5
IN
DS ON DRV
(
CC
Loss. The input capacitor has the diffi cult job of
and V
V
)_
IN
= f(Q
2
G
•
CC
I
required to switch the power MOSFETs.
OUT
⎛
⎜
⎝
G(TOP)
DRV
Current. This is the sum of the MOSFET
≈
•
C
CC
RSS
+ Q
OUT
−
1
G(BOT)
V
• •
GS TH
ESR loss, Schottky conduc-
f
(
)
)
+
V
GS TH
1
(
)
2
⎞
⎟
⎠
R loss and
IN
a load step occurs, V
equal to ΔI
resistance of C
charge C
regulator to return V
this recovery time, V
or ringing that would indicate a stability problems. The
I
vide adequate compensation for most applications. 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
specifi cations: V
= 2.5V, V
f = 500kHz and V
First calculate the timing resistor:
Select a standard value of 715k.
Select a standard value of 511k.
Next, choose the feedback resistors:
Select R1 = 31.6k, R2 = 10k.
Select R3 = 20k, R4 = 10k.
For V
an extra pair of R3 and R4 across V
tied to the TRACK2 pin.
TH
R
R
R
R
R
R
pin external components shown in Figure 13 will pro-
ON1
ON2
2
3
1
4
OUT2
=
=
OUT
=
=
2 5
0 6
1 8
0 6
OUT2
to coincidently track V
.
.
.
.
(
LOAD
(
0 7
0 7
generating a feedback error signal used by the
V
V
V
V
.
.
–
OUT
–
V
= 1.8V, I
V
(ESR), where ESR is the effective series
1 3 17
)(
1 2
IN
)(
OUT2
=
=
500
. ΔI
500
= 7V to 28V (15V nominal), V
2 5
1 8
OUT
OUT
OUT
.
.
.
LOAD
to track V
kHz
kHz
V
V
immediately shifts by an amount
OUT1(MAX)
to its steady-state value. During
can be monitored for overshoot
)(
)(
also begins to charge or dis-
10
10
pF
pF
OUT1
OUT1
)
)
OUT1
=
= I
=
.
714
at start-up, connect
514
OUT2(MAX)
with its midpoint
k
k
= 10A,
OUT1
3708fb