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| 型号： | LTC3619BIDD#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 8/20页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC ADJ DL 10DFN |
| 标准包装： | 121 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 2 |
| 输出电压： | 0.6 V ~ 5 V |
| 输入电压： | 2.5 V ~ 5.5 V |
| PWM 型： | 电流模式，混合 |
| 频率 - 开关： | 2.25MHz |
| 电流 - 输出： | 400mA，800mA |
| 同步整流器： | 是 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 10-WFDFN 裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 10-DFN（3x3） |
LTC3619B
OPERATION
The LTC3619B uses a constant-frequency, current mode
architecture. The operating frequency is set at 2.25MHz.
Both channels share the same clock and run in-phase.
The output voltage is set by an external resistor divider
returned to the V FB pins. An error amplifier compares the
divided output voltage with a reference voltage of 0.6V and
regulates the peak inductor current accordingly.
The LTC3619B continuously monitors the input current
of both channels. When the sum of the currents of both
channels exceeds the programmed input current limit set
by an external resistor, R LIM , channel 2 is current limited
while channel 1 remains regulated.
Main Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle
when the V FB voltage is below the reference voltage. The
current into the inductor and the load increases until the
peak inductor current (controlled by I TH ) is reached. The
RS latch turns off the synchronous switch and energy
stored in the inductor is discharged through the bottom
switch (N-channel MOSFET) into the load until the next
clock cycle begins, or until the inductor current begins to
reverse (sensed by the I RCMP comparator).
The peak inductor current is controlled by the internally
compensated I TH voltage, which is the output of the er-
ror amplifier. This amplifier regulates the V FB pin to the
internal 0.6V reference by adjusting the peak inductor
current accordingly.
When the input current limit is engaged, the peak inductor
current will be lowered, which then reduces the switch-
ing duty cycle and V OUT . This allows the input voltage
to stay regulated when its programmed current output
capability is met.
Light Load Operation
The LTC3619B will automatically transition from continu-
ous operation to the pulse-skipping operation when the load
current is low. The inductor current is not fixed during the
pulse-skipping mode which allows the LTC3619B to switch
at constant-frequency down to very low currents, where it
will begin skipping pulses to maintain output regulation.
This mode of operation exhibits low output ripple as well
as low audio noise and reduced RF interference while
providing reasonable low current efficiency.
Dropout Operation
When the input supply voltage decreases toward the
output voltage the duty cycle increases to 100%, which
is the dropout condition. In dropout, the PMOS switch is
turned on continuously with the output voltage being equal
to the input voltage minus the voltage drops across the
internal P-channel MOSFET and the inductor.
An important design consideration is that the R DS(ON)
of the P-channel switch increases with decreasing input
supply voltage (see the Typical Performance Character-
istics section). Therefore, the user should calculate the
worst-case power dissipation when the LTC3619B is used
at 100% duty cycle with low input voltage (see Thermal
Considerations in the Applications Information section).
Soft-Start
In order to minimize the inrush current on the input bypass
capacitor, the LTC3619B slowly ramps up the output volt-
age during start-up. Whenever the RUN1 or RUN2 pin is
pulled high, the corresponding output will ramp from zero
to full-scale over a time period of approximately 950μs. This
prevents the LTC3619B from having to quickly charge the
output capacitor and thus supplying an excessive amount
of instantaneous current.
When the output is loaded heavily, for example, with milli-
farad of capacitance, it may take longer than 950μs to
charge the output to regulation. If the output is still low
after the soft-start time, the LTC3619B will try to quickly
charge the output capacitor. In this case, the input current
limit (after it engages) can prevent excessive amount of
instantaneous current that is required to quickly charge the
output. See the Channel 2 Start-Up from Shutdown curve
in the Typical Performance Characteristics section. After
input current limit is engaged, the output slowly ramps up
to regulation while limited by its 500mA of input current.
3619bfb
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