LTC3419 Datasheet, PDF (11/16 Pages) Linear Technology – Dual Monolithic 600mA Synchronous Step-Down Regulator

LTC3419
APPLICATIONS INFORMATION
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 ΔI_LOAD • ESR, where ESR is the effective series
resistance of C_OUT. ΔI_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 initial output voltage step may not be within the
bandwidth of the feedback loop, so the standard second
order overshoot/DC ratio cannot be used to determine the
phase margin. In addition, feedback capacitors (C_F1 and
C_F2) can be added to improve the high frequency response,
as shown in Figure 1. Capacitor C_F provides phase lead by
creating a high frequency zero with R2 which improves
the phase margin.
The output voltage settling behavior is related to the stability
of the closed-loop system and will demonstrate the actual
overall supply performance. For a detailed explanation of
optimizing the compensation components, including a re-
view of control loop theory, refer to Application Note 76.
In some applications, a more severe transient can be caused
by switching in loads with large (>1μF) input capacitors.
The discharged input capacitors are effectively put in paral-
lel with C_OUT, causing a rapid drop in V_OUT. No regulator
can deliver enough current to prevent this problem if the
switch connecting the load has low resistance and is driven
quickly. The solution is to limit the turn-on speed of the
load switch driver. A Hot Swap™ controller is designed
specifically for this purpose and usually incorporates cur-
rent limiting, short-circuit protection, and soft-starting.
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. Percent efficiency can
be expressed as:
% Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc., are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, four sources usually account for the losses in
LTC3419 circuits: 1) V_IN quiescent current, 2) switching
losses, 3) I²R losses, 4) other system losses.
1. The V_IN current is the DC supply current given in the
Electrical Characteristics which excludes MOSFET
driver and control currents. V_IN current results in a
small (<0.1%) loss that increases with V_IN, even at
no load.
2. The switching current is the sum of the MOSFET driver
and control currents. The MOSFET driver current re-
sults from switching the gate capacitance of the power
MOSFETs. Each time a MOSFET gate is switched from
low to high to low again, a packet of charge dQ moves
from V_IN to ground. The resulting dQ/dt is a current out
of V_IN that is typically much larger than the DC bias cur-
rent. In continuous mode, I_GATECHG = f_O(Q_T + Q_B), where
Q_T and Q_B are the gate charges of the internal top and
bottom MOSFET switches. The gate charge losses are
proportional to V_IN and thus their effects will be more
pronounced at higher supply voltages.
3. I²R losses are calculated from the DC resistances
of the internal switches, R_SW, and external inductor,
R_L. In continuous mode, the average output current
flows through inductor L, but is “chopped” between
the internal top and bottom switches. Thus, the series
resistance looking into the SW pin is a function of both
top and bottom MOSFET R_DS(ON) and the duty cycle
(DC) as follows:
R_SW = (R_DS(ON)TOP) • (DC) + (R_DS(ON)BOT) • (1– DC)
Hot Swap is a trademark of Linear Technology Corporation.
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