LTC3714_15 Datasheet, PDF (20/28 Pages) Linear Technology – Intel Compatible, Wide Operating Range, Step-Down Controller with Internal Op Amp

LTC3714
Applications Information
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 LTC3714 circuits:
1. DC I²R losses. These arise from the resistances of
the 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_DS(ON), then the resistance of one MOSFET can
simply be summed with the resistances of L and the
board traces to obtain the DC I²R loss. For example, if
R_DS(ON) = 0.01Ω and R_L = 0.005Ω, the loss will range
from 15mW up to 1.5W 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
capacitance, among other factors. The loss is significant
at input voltages above 20V and can be estimated from:
Transition Loss ≅ (1.7A^–1) V_IN2 I_OUT C_RSS f
3. INTV_CC current. This is the sum of the MOSFET driver
and control currents. This loss can be reduced by sup-
plying INTV_CC current through the EXTV_CC pin from a
high efficiency source, such as an output derived boost
network or alternate supply if available.
4. C_IN loss. The input capacitor has the difficult job of
filtering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I²R loss
and sufficient capacitance to prevent the RMS current
from causing additional upstream losses in fuses or
batteries.
Other losses, including C_OUT ESR loss, Schottky diode
D1 conduction loss during dead time and inductor core
loss generally account for less than 2% additional loss.
20
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_OUT immediately shifts by an amount
equal to ∆I_LOAD • (ESR), where ESR is the effective series
resistance of C_OUT. ∆I_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 problem. The I_TH
pin external components shown in Figure 8 will provide
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 follow-
ing specifications: V_IN = 7V to 24V (15V nominal),
V_OUT = 1.15V ±100mV, I_OUT(MAX) = 15A, f = 300kHz. First,
calculate the timing resistor with V_ON = V_OUT:
R_ON
=
1
(^300kHz)
(^10pF)
=
330k
and choose the inductor for about 40% ripple current at
the maximum V_IN:
L
=
1.15V
(^300kHz ) (^0.4) (^15A )
⎛⎝⎜_1−
1.15V
24V
⎞
⎠⎟
=
0.6µH
Choosing a standard value of 0.68µH results in a maximum
ripple current of:
ΔI_L
=
1.15V
(^300kHz ) (^0.68µH)
⎛⎝⎜_1–
1.15V
24V
⎞
⎠⎟
=
5.4A
3714f