LTC3605_15 Datasheet, PDF (13/22 Pages) Linear Technology – 15V, 5A Synchronous Step-Down Regulator

LTC3605
Operation
having a rise time of 1µs to 10µs will produce output volt-
age and ITH pin waveforms that will give a sense of the
overall loop stability without breaking the feedback loop.
Switching regulators take several cycles to respond to a
step in load current. When a load step occurs, V_OUT im-
mediately shifts by an amount equal to DI_LOAD • ESR, where
ESR is the effective series resistance of C_OUT. DI_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 initial output voltage step may not be within the band-
width of the feedback loop, so the standard second order
overshoot/DC ratio cannot be used to determine phase
margin. The gain of the loop increases with the R and the
bandwidth of the loop increases with decreasing C. If R
is increased by the same factor that C is decreased, the
zero frequency will be kept the same, thereby keeping the
phase the same in the most critical frequency range of the
feedback loop. In addition, a feedforward capacitor, C_FF ,
can be added to improve the high frequency response, as
shown in Figure 1. Capacitor C_FF 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
review of control loop theory, refer to Linear Technology
Application Note 76.
In some applications, a more severe transient can be caused
by switching in loads with large (>10µF) 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
current 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, three main sources usually account for most of
the losses in LTC3605 circuits: 1) I²R losses, 2) switching
and biasing losses, 3) other losses.
1. 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 power MOSFETs. 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)
The R_DS(ON) for both the top and bottom MOSFETs can be
obtained from the Typical Performance Characteristics
curves. Thus to obtain I²R losses:
I²R losses = I_OUT2(R_SW + R_L)
2. The INTV_CC current is the sum of the power MOSFET
driver and control currents. The power MOSFET driver
current results from switching the gate capacitance of
the power MOSFETs. Each time a power MOSFET gate is
switched from low to high to low again, a packet of charge
dQ moves from INTV_CC to ground. The resulting dQ/dt
is a current out of INTV_CC that is typically much larger
than the DC control bias current. In continuous mode,
I_GATECHG = f(Q_T + Q_B), where Q_T and Q_B are the gate
charges of the internal top and bottom power MOSFETs
and f is the switching frequency. Since INTV_CC is a low
dropout regulator output powered by V_IN, its power
loss equals:
P_LDO = V_IN • I_INTVCC
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