LTC3606B_15 Datasheet, PDF (12/20 Pages) Linear Technology – 800mA Synchronous Step-Down DC/DC with Average Input Current Limit

LTC3606B
APPLICATIONS INFORMATION
Setting the Output Voltage
The LTC3606B regulates the V_FB pin to 0.6V during
regulation. Thus, the output voltage is set by a resistive
divider, Figure 2, according to the following formula:
V_OUT = 0.6V
1+ ^R2
R1
(2)
Keeping the current small (< 10μA) in these resistors
maximizes efficiency, but making it too small may allow
stray capacitance to cause noise problems or reduce the
phase margin of the error amp loop.
To improve the frequency response of the main control
loop, a feedback capacitor (C_F) may also be used. Great
care should be taken to route the V_FB line away from noise
sources, such as the inductor or the SW line.
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 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_F)
can be added to improve the high frequency response, as
shown in Figure 2. 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 review 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 parallel
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 percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four sources usually account for the losses in
LTC3606B 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.
3606bfb
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