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参数资料
| 型号： | LTC3785EUF-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 15/20页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST PWM VM 24-QFN |
| 标准包装： | 91 |
| PWM 型： | 电压模式 |
| 输出数： | 2 |
| 频率 - 最大： | 1MHz |
| 占空比： | 99% |
| 电源电压： | 2.7 V ~ 10 V |
| 降压： | 是 |
| 升压： | 是 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 24-WFQFN 裸露焊盘 |
| 包装： | 管件 |
LTC3785-1
APPLICATIONS INFORMATION
500 ? 10 3 ? 3 ? 40 ? ( 3.3 )
3.3 ? ( 10 – 3.3 ) ? 100
EFFICIENCY CONSIDERATIONS
The percentage ef?ciency 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 ef?ciency and which change would
produce the most improvement. Although all dissipative
elements in circuits produce losses, four main sources
account for most of the losses in LTC3785-1 application
circuits:
1. DC I 2 R losses. These arise from the resistances of the
MOSFETs, sensing resistor (if used), inductor and PC
board traces and cause the ef?ciency to drop at high
output currents.
2. Transition loss. This loss arises from the brief voltage
transition time of switch A or switch C. It depends upon
the switch voltage, inductor current, driver strength and
MOSFET capacitance, among other factors.
Transition Loss ~ V SW2 ? I L ? C RSS ? f
where C RSS is the reverse transfer capacitance.
3. C IN and C OUT loss. The input capacitor has the dif?cult
job of ?ltering the large RMS input current to the regula-
tor in buck mode. The output capacitor has the more
dif?cult job of ?ltering the large RMS output current in
boost mode. Both C IN and C OUT are required to have
low ESR to minimize the AC I 2 R loss and suf?cient
capacitance to prevent the RMS current from causing
additional upstream losses in fuses or batteries.
5. V CC regulator loss. In applications where the input
voltage is above 5V, such as two Li-Ion cells, the V CC
regulator will dissipate some power due the differential
voltage and the average output current to the drive the
gates of the output switches. The V CC pin can be driven
directly from a high ef?ciency external 5V source if
desired to incrementally improve overall ef?ciency at
lighter loads.
DESIGN EXAMPLE
As a design example, assume V IN = 2.7V to 10V (3.6V
nominal Li-Ion with 9V adapter), V OUT = 3.3V (5%),
I OUT(MAX) = 3A and f = 500kHz.
Determine the Inductor Value
Setting the Inductor Ripple to 40% and using the equations
in the Inductor Selection section gives:
L > ( 2.7 ) 2 ? ( 3.3 – 2.7 ) ? 100 = 0.67μH
2
L > = 3.7μH
500 ? 10 3 ? 3 ? 40 ? 10
So the worst-case ripple for this application is during buck
mode so a standard inductor value of 3.3μH is chosen.
Determine the Proper Inductor Type Selection
The highest inductor current is during boost mode and
is given by:
4. Other losses. Optional Schottky diodes D1 and D2 are
responsible for conduction losses during dead time
and light load conduction periods. Core loss is the
I L(MAX _ AV) =
V OUT ?I OUT
V IN ? η
predominant inductor loss at light loads. Turning on
where η = estimated ef?ciency in this mode (use 80%).
switch C causes reverse recovery current loss in boost
mode. When making adjustments to improve ef?ciency,
the input current is the best indicator of changes in
I L(MAX _ AV) =
3.3 ? 3
2.7 ? 0.8
= 4.6A
ef?ciency. If you make a change and the input current
decreases, then the ef?ciency has increased. If there
is no change in input current, then there is no change
in ef?ciency.
37851fa
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