Datasheet

LTC3708
22
3708fb
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:
TransitionLoss
VI C f
R
DRV V V
IN OUT RSS
DS ON DRV
CC GS TH GS TH
≈
−
+
⎛
⎝
⎜
⎞
⎠
⎟
(.)• • • • •
()_
() ()
05
11
2
3. DRV
CC
and V
CC
Current. This is the sum of the MOSFET
driver and control currents. The driver current supplies the
gate charge Q
G
required to switch the power MOSFETs.
This current is typically much larger than the control circuit
current. In continuous mode operation:
I
GATECHG
= f(Q
G(TOP)
+ Q
G(BOT)
)
4. C
IN
Loss. The input capacitor has the diffi cult job of
fi ltering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I
2
R loss and
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
IN
loss over the single phase solutions.
Other losses, including C
OUT
ESR loss, Schottky conduc-
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
a load step occurs, V
OUT
immediately shifts by an amount
equal to ΔI
LOAD
(ESR), where ESR is the effective series
resistance of C
OUT
. ΔI
LOAD
also begins to charge or dis-
charge C
OUT
generating a feedback error signal used by the
regulator to return V
OUT
to its steady-state value. During
this recovery time, V
OUT
can be monitored for overshoot
or ringing that would indicate a stability problems. The
I
TH
pin external components shown in Figure 13 will pro-
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
IN
= 7V to 28V (15V nominal), V
OUT1
= 2.5V, V
OUT2
= 1.8V, I
OUT1(MAX)
= I
OUT2(MAX)
= 10A,
f = 500kHz and V
OUT2
to track V
OUT1
.
First calculate the timing resistor:
R
V
V kHz pF
k
ON1
25
0 7 500 10
714=
()( )()
=
.
.
Select a standard value of 715k.
R
V
V kHz pF
k
ON2
18
0 7 500 10
514=
()( )()
=
.
.
Select a standard value of 511k.
Next, choose the feedback resistors:
R
R
V
V
1
2
25
06
1317==
.
.
–.
Select R1 = 31.6k, R2 = 10k.
R
R
V
V
3
4
18
06
12==
.
.
–
Select R3 = 20k, R4 = 10k.
For V
OUT2
to coincidently track V
OUT1
at start-up, connect
an extra pair of R3 and R4 across V
OUT1
with its midpoint
tied to the TRACK2 pin.