LTC3879_15 Datasheet, PDF (19/28 Pages) Linear Technology – Fast, Wide Operating Range No RSENSE Step-Down Controller

LTC3879
APPLICATIONS INFORMATION
Other losses, which include the C_OUT ESR loss, bottom
MOSFET reverse recovery loss and inductor core loss
generally account for less than 2% additional loss.
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased. If there is no change in input
current there is no change in efficiency.
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_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 discharge 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
problem. The I_TH pin external components shown in the
Design Example will provide adequate compensation for
most applications.
A rough compensation check can be made by calculating
the gain crossover frequency, f_GCO. g_m(EA) is the error
amplifier transconductance, R_C is the compensation re-
sistor and feedback divider attenuation is assumed to be
0.6V/V_OUT. This equation assumes that no feed-forward
compensation is used on feedback and that C_OUT sets the
dominant output pole.
f_GCO
=
g_m(EA) • R_C
•
I_LIMIT
1.6
•
2•
π
1
• C_OUT
•
0.6
V_OUT
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.
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_P-P or greater. This ripple voltage is equal
to R_DS(ON) typical at 25°C • I_P-P.
Design Example
Figure 10 is a power supply design example with the fol-
lowing specifications: V_IN = 4.5V to 28V (12V nominal),
V_OUT = 1.2V ±5%, I_OUT(MAX) = 15A and f = 400kHz. Start
by calculating the timing resistor, R_ON:
R_ON
=
0.7V
•
1.2V
400kHz
•
10pF
=
429k
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_IN using
the adjusted operating frequency:
L
=
1.2V
396kHz • 0.35
•
15A
⎛
⎝⎜
1–
1.2 ⎞
28 ⎠⎟
=
0.55μH
Select 0.56μH which is the nearest value.
The resulting maximum ripple current is:
ΔI_L
=
1.2V
396kHz • 0.56μH
⎛
⎝⎜
1–
1.2V
28V
⎞
⎠⎟
=
5.1A
Choose the synchronous bottom MOSFET switch and
calculate the V_RNG current limit set-point. To calculate
V_RNG and V_DS, the ρτ term normalization factor (unity
3879f
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