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参数资料
| 型号： | LTC3822EGN-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 12/24页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 16-SSOP |
| 标准包装： | 100 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 850kHz |
| 占空比： | 99% |
| 电源电压： | 2.75 V ~ 4.5 V |
| 降压： | 是 |
| 升压： | 无 |
| 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： | 16-SSOP（0.154"，3.90mm 宽） |
| 包装： | 管件 |
| 产品目录页面： | 1336 (CN2011-ZH PDF) |
LTC3822-1
APPLICATIONS INFORMATION
The typical LTC3822-1 application circuit is shown on
the front page of this data sheet. External component
selection for the controller is driven by the load require-
A reasonable starting point is setting ripple current I RIPPLE
to be 40% of I OUT(MAX) . Rearranging the above equation
yields:
I OUT ( MAX )
ment and begins with the selection of the inductor and
the power MOSFETs.
R DS ( ON ) MAX =
5 ? V SENSE ( MAX )
?
6
for Dutyy Cycle < 20%
5 ? V SENSE ( MAX )
R DS ( ON ) MAX = ? SF ?
I OUT ( MAX )
Power MOSFET Selection
The LTC3822-1’s controller requires external N-chan-
nel power MOSFETs for the topside (main) and bottom
(synchronous) switches. The main selection criteria for
the power MOSFETs are the breakdown voltage V BR(DSS) ,
threshold voltage V GS(TH) , on-resistance R DS(ON) , reverse
transfer capacitance C RSS , turn-off delay t D(OFF) and the
total gate charge Q G .
The gate drive voltage is usually the input supply voltage.
Since the LTC3822-1 is designed for operation at low input
voltages, a sublogic level MOSFET (R DS(ON) guaranteed at
V GS = 2.5V) is required.
The topside MOSFET’s on-resistance is chosen based on
the required load current. The maximum average load
current I OUT(MAX) is equal to the peak inductor current
minus half the peak-to-peak ripple current I RIPPLE . The
However, for operation above 20% duty cycle, slope
compensation has to be taken into consideration to select
the appropriate value of R DS(ON) to provide the required
amount of load current:
6
where SF is a scale factor whose value is obtained from
the curve in Figure 1.
These must be further derated to take into account the
signi?cant variation in on-resistance with temperature. The
following equation is a good guide for determining the re-
quired R DS(ON)MAX at 25°C (manufacturer’s speci?cation),
allowing some margin for variations in the LTC3822-1 and
external component values:
LTC3822-1’s current comparator monitors the drain-to-
source voltage V DS of the top MOSFET, which is sensed
between the V IN and SW pins. The peak inductor current
R DS ( ON ) MAX =
5
6
? 0 . 9 ? SF ?
? V SENSE(MAX )
I OUT ( MAX ) ? ρ T
? V SENSE ( MAX ) I
R DS ( ON )
is limited by the current threshold, set by the voltage on
the I TH pin, of the current comparator. The voltage on the
I TH pin is internally clamped, which limits the maximum
current sense threshold Δ V SENSE(MAX) to approximately
125mV when IPRG is ?oating (82mV when IPRG is tied
low; 200mV when IPRG is tied high).
The output current that the LTC3822-1 can provide is
given by:
I OUT ( MAX ) = – RIPPLE
2
The ρ T is a normalizing term accounting for the temperature
variation in on-resistance, which is typically about 0.4%/°C,
as shown in Figure 2. Junction-to-case temperature Δ T JC
is about 10°C in most applications. For a maximum ambi-
ent temperature of 70°C, using ρ 80°C ≈ 1.3 in the above
equation is a reasonable choice.
The power dissipated in the MOSFETs strongly depends
on their respective duty cycles and load current. When
the LTC3822-1 is operating in continuous mode, the duty
cycles for the MOSFETs are:
where I RIPPLE is the inductor peak-to-peak ripple current
(see Inductor Value Calculation).
Top MOSFET Duty Cycle =
V OUT
V IN
V IN OUT
Bottom MOSFET Duty Cycle =
– V
V IN
38221f
12
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