LTC3879 Linear Technology Corporation, LTC3879 Datasheet - Page 19

• Manufacturer Part Number: LTC3879
• Description: Wide Operating Range No RSENSE Step-Down Controller
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
Other losses, which include the C
MOSFET reverse recovery loss and inductor core loss
generally account for less than 2% additional loss.
When making adjustments to improve effi ciency, the input
current is the best indicator of changes in effi ciency. If you
make a change and the input current decreases, then the
effi ciency has increased. If there is no change in input
current there is no change in effi ciency.
Checking Transient Response
The regulator loop response can be checked by look-
ing at the load transient response. Switching regulators
take several cycles to respond to a step in load current.
When a load step occurs, V
amount equal to ΔI
series resistance of C
or discharge C
used by the regulator to return V
value. During this recovery time, V
for overshoot or ringing that would indicate a stability
problem. The I
Design Example will provide adequate compensation for
most applications.
A rough compensation check can be made by calculating
the gain crossover frequency, f
amplifi er transconductance, R
sistor and feedback divider attenuation is assumed to be
0.6V/V
compensation is used on feedback and that C
dominant output pole.
As a rule of thumb the gain crossover frequency should be
less than 20% of the switching frequency. For a detailed
explanation of switching control loop theory see Applica-
tion Note 76.
f
GCO
OUT
=
. This equation assumes that no feed-forward
g
m EA
(
TH
OUT
)
•
pin external components shown in the
R
LOAD
, generating a feedback error signal
C
OUT
•
I
(ESR), where ESR is the effective
LIMIT
1 6
. ΔI
.
OUT
LOAD
•
C
2
immediately shifts by an
GCO
is the compensation re-
• •
OUT
OUT
also begins to charge
OUT
π
. g
1
C
to its steady-state
ESR loss, bottom
m(EA)
can be monitored
OUT
•
OUT
is the error
V
0 6
OUT
.
sets the
High Switching Frequency Operation
Special care should be taken when operating at switch-
ing frequencies greater than 800kHz. At high switching
frequencies there may be an increased sensitivity to
PCB noise which may result in off-time variation greater
than normal. This off-time instability can be prevented
in several ways. First, carefully follow the recommended
layout techniques. Second, use 2μF or more of X5R or
X7R ceramic input capacitance per Amp of load current.
Third, if necessary, increase the bottom MOSFET ripple
voltage to 30mV
to R
Design Example
Figure 10 is a power supply design example with the fol-
lowing specifi cations: V
V
by calculating the timing resistor, R
Select the nearest standard resistor value of 432k for a
nominal operating frequency of 396kHz. Set the inductor
value to give 35% ripple current at maximum V
the adjusted operating frequency:
Select 0.56μH which is the nearest value.
The resulting maximum ripple current is:
Choose the synchronous bottom MOSFET switch and
calculate the V
V
OUT
RNG
R
L
ΔI
DS(ON)
ON
=
= 1.2V ±5%, I
L
and V
396
=
=
396
0 7
kHz
typical at 25°C • I
.
DS
kHz
V
1 2
, the ρτ term normalization factor (unity
• .
•
1 2
RNG
.
P-P
0 35 15
400
.
• .
V
1 2
V
0 56
.
OUT(MAX)
or greater. This ripple voltage is equal
current limit set-point. To calculate
kHz
•
V
μH
IN
•
A
10
= 4.5V to 28V (12V nominal),
⎛
⎝ ⎜
⎛
⎝ ⎜
1
P-P
pF
1
= 15A and f = 400kHz. Start
–
–
1 2
.
28
=
1 2
28
.
.
429
V
V
ON
⎞
⎠ ⎟
⎞
⎠ ⎟
=
k
:
=
0 55
LTC3879
5 1
.
.
A
μH
IN
19
using
3879f