LTC3863 Datasheet, PDF (21/36 Pages) Linear Technology – 60V Low IQ Inverting DC/DC Controller Wide Operating VIN Range: 3.5V to 60V

LTC3863
APPLICATIONS INFORMATION
If the duty cycle falls below what can be accommodated
by the minimum on-time, the controller will skip cycles.
However, the output voltage will continue to regulate.
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
the dominant contributors and therefore where efficiency
improvements can be made. Percent efficiency can be
expressed as:
% Efficiency = 100% - (L1+L2+L3+…)
where L1, L2, L3, etc., are the individual losses as a per-
centage of input power.
Although all dissipative elements in the circuit produce
losses, four main sources account for most of the losses
in LTC3863 application circuits.
1. Conduction Loss: Conduction losses result from the
P-channel MOSFET R_DS(ON), inductor resistance DCR,
the current sense resistor R_SENSE, and input and output
capacitor ESR. The current through DCR is continuous.
The currents through both the P-channel MOSFET and
Schottky diode are discontinuous. The following equa-
tion may be used to determine the total conduction loss
(P_COND) in continuous conduction mode:
P_COND
≈



I_OUT2
(1– D)²
+
∆I_L2
12



( ) _• ^{R}_DCR ^+D•
R_DS(ON) +R_SENSE +R_ESR(CIN)


^{+(1–D) •RESR(COUT)}

2. Transition Loss: Transition loss of the P-channel
MOSFET becomes significant only when operating
at high input voltages (typically 20V or greater.) The
P‑channel transition losses (P_MOSTRL) can be deter-
mined from the following equation:
( ) _{PPMOSTRL =} ^{f •C}_MILLER ^•
V_IN + | V_OUT | +V_D
2
2
( )
•
I_OUT
1– D
•



V_IN
–
R_DN
V_CAP –
V_MILLER
+
R_UP
V_MILLER



3. Gate Charging Loss: Charging and discharging the gate
of the MOSFET will result in an effective gate charg-
ing current. Each time the P-channel MOSFET gate is
switched from low to high and low again, a packet of
charge, dQ, moves from the capacitor across V_IN – V_CAP
and is then replenished from ground by the internal V_CAP
regulator. The resulting dQ/dt current is a current out
of V_IN flowing to ground. The total power loss in the
controller including gate charging loss is determined
by the following equation:
P_CNTRL = V_IN • (I_Q + f • Q_G(PMOSFET))
4. Schottky Loss: The Schottky loss is independent of
duty factors. The critical component is the Schottky
forward voltage as a function of junction temperature
and current. The Schottky power loss is given by the
equation:
P_DIODE = I_OUT • V_D(IOUT,TJ)
When making adjustments to improve efficiency, the in-
put current is the best indicator of changes in efficiency.
If changes cause the input current to decrease, then the
efficiency has increased. If there is no change in input
current, there is no change in efficiency.
For more information www.linear.com/3863
3863f
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