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参数资料
| 型号： | LTC3605IUF#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 7.5 A SWITCHING REGULATOR, 4000 kHz SWITCHING FREQ-MAX, PQCC24 |
| 封装： | 4 X 4 MM, LEAD FREE, PLASTIC, MO-220WGGD, QFN-24 |
| 文件页数： | 5/22页 |
| 文件大小： | 257K |
| 代理商： | LTC3605IUF#TRPBF |
LTC3605
13
3605fb
steady-state value. During this recovery time, VOUT 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, CFF,
can be added to improve the high frequency response, as
shown in Figure 1. Capacitor CFF 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 parallel with COUT, causing a rapid drop in VOUT. No
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 specically for this purpose and usually in-
corporates current limiting, short-circuit protection and
soft-starting.
Efciency Considerations
The percent efciency 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 efciency and which change would
produce the most improvement. Percent efciency can
be expressed as:
% Efciency = 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, RSW, and external inductor, RL.
In continuous mode, the average output current ows
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 RDS(ON) and the duty
cycle (DC) as follows:
RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1-DC)
The RDS(ON)forboththetopandbottomMOSFETscanbe
obtained from the Typical Performance Characteristics
curves. Thus to obtain I2R losses:
I2R losses = IOUT2(RSW + RL)
2. The INTVCC 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 INTVCC to ground. The resulting dQ/dt
is a current out of INTVCC that is typically much larger
than the DC control bias current. In continuous mode,
IGATECHG = f(QT + QB), where QT and QB are the gate
charges of the internal top and bottom power MOSFETs
and f is the switching frequency. Since INTVCC is a low
dropout regulator output powered by VIN, its power
loss equals:
PLDO = VIN IINTVCC
Refer to the IINTVCC vs Frequency curve in the Typical
Performance Characterics for typical INTVCC current at
various frequencies.
3. Other “hidden” losses such as transition loss and cop-
per trace and internal load resistances can account for
additional efciency degradations in the overall power
OPERATION
Hot Swap is a trademark of Linear Technology Corporation.
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