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参数资料
| 型号： | LTC3809IDD-1#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 12/24页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 10-DFN |
| 标准包装： | 2,500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 600kHz |
| 占空比： | 100% |
| 电源电压： | 2.75 V ~ 9.8 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： | 10-WFDFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
LTC3809-1
APPLICATIONS INFORMATION
The typical LTC3809-1 application circuit is shown in Figure
8. External component selection for the controller is driven
by the load requirement and begins with the selection of
the inductor and the power MOSFETs.
Power MOSFET Selection
where I RIPPLE is the inductor peak-to-peak ripple current
(see Inductor Value Calculation).
A reasonable starting point is setting ripple current I RIPPLE
to be 40% of I OUT(MAX) . Rearranging the above equation
yields:
5 Δ V SENSE ( MAX )
R DS ( ON ) MAX = ?
I OUT ( MAX )
The LTC3809-1’s controller requires two external power
MOSFETs: a P-channel MOSFET for the topside (main)
switch and a N-channel MOSFET for the bottom (synchro-
6
for Duty C y cle < 20%
nous) switch. 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
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:
R DS ( ON ) MAX = ? SF ?
charge Q G .
The gate drive voltage is the input supply voltage. Since
the LTC3809-1 is designed for operation down to low input
5
6
Δ V SENSE(MAX )
I OUT ( MAX )
5 Δ V SENSE ( MAX )
I OUT ( MAX ) ? ρ T
R DS ( ON ) MAX = ? 0 . 9 ? SF ?
voltages,asublogiclevelMOSFET(R DS(ON) guaranteedat
V GS = 2.5V) is required for applications that work close to
this voltage. When these MOSFETs are used, make sure that
the input supply to the LTC3809-1 is less than the absolute
maximum MOSFET V GS rating, which is typically 8V.
The P-channel 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
LTC3809-1’s current comparator monitors the drain-to-
source voltage V DS of the top P-channel MOSFET, which
is sensed between the V IN and SW pins. The peak inductor
current 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 (85mV when IPRG is tied
low; 204mV when IPRG is tied high).
The output current that the LTC3809-1 can provide is
given by:
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 LTC3809-1 and
external component values:
6
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 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 N-channel MOSFET’s on resistance is chosen based
on the short-circuit current limit (I SC ). The LTC3809-
Δ V SENSE ( MAX ) I RIPPLE
R DS ( ON )
I OUT ( MAX ) =
–
2
1’s short-circuit current limit comparator monitors the
drain-to-source voltage V DS of the bottom N-channel
MOSFET, which is sensed between the GND and SW pins.
38091fc
12
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