参数资料
| 型号： | LTC3634IFE#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 18/28页 |
| 文件大小： | 0K |
| 描述： | IC CONV DDR DDR2 DDR3 28TSSOP |
| 标准包装： | 50 |
| 应用： | 转换器，DDR，DDR2，DDR3 |
| 输入电压： | 1.4 V ~ 15 V |
| 输出数： | 2 |
| 输出电压： | 0.6 V ~ 3 V |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 28-SOIC（0.173"，4.40mm 宽）裸露焊盘 |
| 供应商设备封装： | 28-TSSOP 裸露焊盘 |
| 包装： | 管件 |
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LTC3634
APPLICATIONS INFORMATION
V IN(MIN) =
(
)
MinimumOff-Time/On-TimeConsiderations
The minimum off-time is the smallest amount of time that
the LTC3634 can turn on the bottom power MOSFET, trip
the current comparator and turn the power MOSFET back
off. This time is typically 40ns. For the controlled on-time
control architecture, the minimum off-time limit imposes
a maximum duty cycle of:
DC MAX = 1– f ? (t OFF(MIN) + 2 ? t DEAD )
where f is the switching frequency, t DEAD is the nonoverlap
time of the switches, or dead time (typically 15ns), and
t OFF(MIN) is the minimum off-time. If the maximum duty
cycle is surpassed, due to a decreasing input voltage
for example, the output will drop out of regulation. The
minimum input voltage to avoid this dropout condition is:
V OUT
1 ? f ? t OFF(MIN) + 2 ? t DEAD
Conversely, the minimum on-time is the smallest dura-
tion of time in which the top power MOSFET can be in
its ON state. This time is typically 20ns. In continuous
mode operation, the minimum on-time limit imposes a
minimum duty cycle of:
DC MIN = (f ? t ON(MIN) )
where t ON(MIN) is the minimum on-time. As the equation
shows, reducing the operating frequency will alleviate the
minimum duty cycle constraint.
When the regulator output is sinking current, the effective
minimum on-time of the converter will be increased by the
non-overlap time of the power MOSFETs (or the “dead-
time”) during each SW node transition. This “dead-time”
is nominally 15ns, so when sinking current, the minimum
on-time is effectively 15ns + 15ns + 20ns = 50ns.
If the minimum on-time constraint is violated, the converter
will automatically reduce its own switching frequency in
order to maintain output regulation. Once the converter
reduces its switching frequency, the phase information
is lost and the two channels will switch asynchronously.
Furthermore, the regulator may need to be compensated
more conservatively due to the lower switching frequency.
MODE/SYNC Operation
The MODE/SYNC pin is a multipurpose pin allowing both
mode selection and operating frequency synchronization.
Floating this pin or connecting it to INTV CC enables Burst
Mode operation on channel 1 for superior efficiency at
light load currents at the expense of slightly higher out-
put voltage ripple. When the MODE/SYNC pin is tied to
ground, forced continuous mode operation is selected,
creating the lowest fixed output ripple at the expense of
light load efficiency.
The LTC3634 will detect the presence of the external
clock signal on the MODE/SYNC pin and synchronize the
internal oscillator to the phase and frequency of the in-
coming clock. The presence of an external clock will place
both regulators into forced continuous mode operation.
Although the R T resistor is not strictly necessary when
synchronizing to an external clock, it is recommended to
use a R T resistor that matches the nominal external clock
frequency in order to keep the switching regulator biased
correctly whenever the external clock signal is suddenly
removed or reapplied.
Channel 1 Output Voltage Tracking and Soft-Start
The LTC3634 allows the user to control the output voltage
ramp rate of channel 1 by means of the TRACKSS pin.
From 0 to 0.6V, the TRACKSS voltage will override the
internal 0.6V reference input to the error amplifier, thus
regulating the feedback voltage to that of the TRACKSS
pin. When TRACKSS is above 0.6V, tracking is disabled
and the feedback voltage will regulate to the internal
reference voltage.
The voltage at the TRACKSS pin may be driven from an
external source, or alternatively, the user may leverage
the internal 1.4μA pull-up current source to implement
a soft-start function by connecting an external capacitor
(C SS ) from the TRACKSS pin to ground. The relationship
between output rise time and TRACKSS capacitance is
given by:
t SS = 430000Ω ? C SS
3634fb
18
For more information www.linear.com/LTC3634
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