LTC3863_15 Datasheet, PDF (21/38 Pages) Linear Technology – 60V Low IQ Inverting DC/DC Controller

LTC3863
Applications Information
V_IN Undervoltage Lockout (UVLO)
The LTC3863 is designed to accommodate applications
requiring widely varying power input voltages from
3.5V to 60V. To accommodate the cases where V_IN
drops significantly once in regulation, the LTC3863 is
guaranteed to operate down to a V_IN of 3.5V over the
full temperature range.
The implications of both the UVLO rising and UVLO falling
specifications must be carefully considered for low V_IN
operation. The UVLO threshold with V_IN rising is typi-
cally 3.5V (with a maximum of 3.8V) and UVLO falling is
typically 3.25V (with a maximum of 3.5V). The operating
input voltage range of the LTC3863 is guaranteed to be
3.5V to 60V over temperature, but the initial V_IN ramp
must exceed 3.8V to guarantee start-up.
Minimum On-Time Considerations
The minimum on-time, t_ON(MIN), is the smallest time
duration that the LTC3863 is capable of turning on the
power MOSFET, and is typically 220ns. It is determined
by internal timing delays and the gate charge required to
turn on the MOSFET. Low duty cycle applications may
approach this minimum on-time limit, so care should be
taken to ensure that:
( ^{( )} ) ^{tON(MIN) < f •}
| V_OUT | +V_D
V_IN(MAX)+ | V_OUT | +V_D
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



( ) _•
_{RDCR +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_PMOSTRL) 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



For more information www.linear.com/3863
3863fa
21