参数资料
| 型号： | LT1913IDD#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 16/24页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 3.5A 10DFN |
| 标准包装： | 2,500 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.79 V ~ 25 V |
| 输入电压： | 3.6 V ~ 25 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 200kHz ~ 2.4MHz |
| 电流 - 输出： | 3.5A |
| 同步整流器： | 无 |
| 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： | 带卷 (TR) |
| 供应商设备封装： | 10-DFN（3x3） |
LT1913
APPLICATIONS INFORMATION
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
on input and output voltages, and on the arrangement of
RUN
15k
I L
1A/DIV
the boost circuit. The minimum load generally goes to
zero once the circuit has started. Figure 5 shows a plot
of minimum load to start and to run as a function of input
0.22μF
RUN/SS
GND
V RUN/SS
2V/DIV
V OUT
2V/DIV
voltage. In many cases the discharged output capacitor
will present a load to the switcher, which will allow it to
2ms/DIV
1913 F06
start. The plots show the worst-case situation where V IN
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to V IN .
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
300mV above V OUT . At higher load currents, the inductor
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT1913, requiring a higher
input voltage to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT1913,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC ?lter to
create a voltage ramp at this pin. Figure 6 shows the start-
up and shut-down waveforms with the soft-start circuit.
By choosing a large RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply 20μA when the RUN/SS
pin reaches 2.5V.
Synchronization
Synchronizing the LT1913 oscillator to an external fre-
quency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square
wave amplitude should have valleys that are below 0.3V
and peaks that are above 0.8V (up to 6V).
The LT1913 may be synchronized over a 250kHz to 2MHz
range. The R T resistor should be chosen to set the LT1913
switching frequency 20% below the lowest synchronization
Figure 6. To Soft-Start the LT1913, Add a Resisitor
and Capacitor to the RUN/SS Pin
input. For example, if the synchronization signal will be
250kHz and higher, the R T should be chosen for 200kHz.
To assure reliable and safe operation the LT1913 will only
synchronize when the output voltage is near regulation
as indicated by the PG ?ag. It is therefore necessary to
choose a large enough inductor value to supply the required
output current at the frequency set by the R T resistor. See
Inductor Selection section. It is also important to note that
slope compensation is set by the R T value: When the sync
frequency is much higher than the one set by R T , the slope
compensation will be signi?cantly reduced which may
require a larger inductor value to prevent subharmonic
oscillation.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT1913 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT1913 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT1913’s
output. If the V IN pin is allowed to ?oat and the RUN/SS
pin is held high (either by a logic signal or because it is
tied to V IN ), then the LT1913’s internal circuitry will pull
its quiescent current through its SW pin. This is ?ne if
your system can tolerate a few mA in this state. If you
ground the RUN/SS pin, the SW pin current will drop to
essentially zero. However, if the V IN pin is grounded while
1913f
16
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