LTC3605 LINER [Linear Technology], LTC3605 Datasheet - Page 13

• Manufacturer Part Number: LTC3605
• Description: 20V, 5A Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

OPERATION
steady-state value. During this recovery time, V
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
can be added to improve the high frequency response, as
shown in Figure 1. Capacitor C
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 parallel with C
regulator can deliver enough current to prevent this prob-
lem, 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 in-
corporates 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 +…)
OUT
, causing a rapid drop in V
FF
provides phase lead by
OUT
OUT
. No
can
FF
,
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 LTC3605A circuits: 1) I
and biasing losses, 3) other losses.
1. I
2. The INTV
3. Other “hidden” losses such as transition loss and cop-
the internal switches, R
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
cycle (DC) as follows:
The R
obtained from the Typical Performance Characteristics
curves. Thus to obtain I
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
is a current out of INTV
than the DC control bias current. In continuous mode,
I
charges of the internal top and bottom power MOSFETs
and f is the switching frequency. Since INTV
dropout regulator output powered by V
loss equals:
Refer to the I
Performance Characterics for typical INTV
various frequencies.
per trace and internal load resistances can account for
additional efficiency degradations in the overall power
2
GATECHG
R losses are calculated from the DC resistances of
R
I
P
2
LDO
SW
R losses = I
DS(ON)
= (R
= V
CC
= f(Q
IN
for both the top and bottom MOSFETs can be
DS(ON)
current is the sum of the power MOSFET
INTVCC
• I
T
OUT
+ Q
INTVCC
TOP)(DC) + (R
2
B
vs Frequency curve in the Typical
(R
CC
), where Q
SW
CC
to ground. The resulting dQ/dt
SW
2
R losses:
that is typically much larger
, and external inductor, R
+ R
L
2
R losses, 2) switching
)
T
DS(ON)
LTC3605A
and Q
DS(ON)
BOT)(1-DC)
B
IN
and the duty
CC
are the gate
, its power
CC
current at
13
is a low
3605af
L
.