Datasheet

LTC3605A
13
3605afg
For more information www.linear.com/LTC3605A
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
operaTion
Table 1. Inductor Selection Table
INDUCTANCE DCR MAX CURRENT DIMENSIONS HEIGHT
Vishay IHLP-2525CZ-01 Series
0.33µH 4.1mW 18A 6.7mm × 7mm 3mm
0.47µH 6.5mW 13.5A
0.68µH 9.4mW 11A
0.82µH 11.8mW 10A
1.0µH 14.2mW 9A
Vishay IHLP-1616BZ-11 Series
0.22µH 4.1mW 12A 4.3mm × 4.7mm 2.0mm
0.47µH 15mW 7A
Toko FDV0620 Series
0.20µH 4.5mW 12.4A 7mm × 7.7mm 2.0mm
0.47µH 8.3mW 9A
1µH 18.3mW 5.7A
NEC/Tokin MLC0730L Series
0.47µH 4.5mW 16.6A 6.9mm × 7.7mm 3.0mm
0.75µH 7.5mW 12.2A
1µH 9mW 10.6A
Cooper HCP0703 Series
0.22µH 2.8mW 23A 7mm × 7.3mm 3.0mm
0.47µH 4.2mW 17A
0.68µH 5.5mW 15A
0.82µH 8mW 13A
1µH 10mW 11A
1.5µH 14mW 9A
TDK RLF7030 Series
1µH 8.8mW 6.4A 6.9mm × 7.3mm 3.2mm
1.5µH 9.6mW 6.1A
2.2µH 12mW 5.4A
Würth Elektronik WE-HC 744312 Series
0.25µH 2.5mW 18A 7mm × 7.7mm 3.8mm
0.47µH 3.4mW 16A
0.72µH 7.5mW 12A
1µH 9.5mW 11A
1.5µH 10.5mW 9A
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