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参数资料
| 型号： | LTC3605EUF#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 13/22页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC ADJ 5A 24QFN |
| 标准包装： | 91 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.6 V ~ 14.9 V |
| 输入电压： | 4 V ~ 15 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 1MHz |
| 电流 - 输出： | 5A |
| 同步整流器： | 是 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 24-WFQFN 裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 24-QFN（4x4） |
| 产品目录页面： | 1335 (CN2011-ZH PDF) |
LTC3605
OPERATION
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 parallel with C OUT , causing a rapid drop in V OUT . 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 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 +…)
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) I 2 R losses, 2) switching
and biasing losses, 3) other losses.
1. I 2 R 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 I 2 R losses:
I 2 R 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
Refer to the I INTVCC vs Frequency curve in the Typical
Performance Characterics for typical INTV CC current at
various frequencies.
3. Other “hidden” losses such as transition loss and cop-
per trace and internal load resistances can account for
additional efficiency degradations in the overall power
Hot Swap is a trademark of Linear Technology Corporation.
3605fc
13
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