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| 型号： | LTC3717EUH-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 14/20页 |
| 文件大小： | 0K |
| 描述： | IC CTRLR STEP-DOWN SYNC 32QFN |
| 标准包装： | 73 |
| 应用： | 控制器，DDR，QDR |
| 输入电压： | 4 V ~ 36 V |
| 输出数： | 1 |
| 输出电压： | 2 V ~ 18 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 32-WFQFN 裸露焊盘 |
| 供应商设备封装： | 32-QFN 裸露焊盘（5x5） |
| 包装： | 管件 |
| 产品目录页面： | 1335 (CN2011-ZH PDF) |
LTC3717-1
APPLICATIO S I FOR ATIO
After the controller has been started and given adequate
time to charge up the output capacitor, C SS is used as a
short-circuit timer. After the RUN/SS pin charges above
4V, if the output voltage falls below 75% of its regulated
value, then a short-circuit fault is assumed. A 1.8 μ A cur-
rent then begins discharging C SS . If the fault condition
persists until the RUN/SS pin drops to 3.5V, then the con-
troller turns off both power MOSFETs, shutting down the
converter permanently. The RUN/SS pin must be actively
pulled down to ground in order to restart operation.
The overcurrent protection timer requires that the soft-
start timing capacitor C SS be made large enough to guar-
antee that the output is in regulation by the time C SS has
reached the 4V threshold. In general, this will depend upon
the size of the output capacitance, output voltage and load
current characteristic. A minimum soft-start capacitor can
be estimated from:
C SS > C OUT V OUT R SENSE (10 – 4 [F/V s])
Generally 0.1 μ F is more than sufficient.
Overcurrent latchoff operation is not always needed or
desired. The feature can be overridden by adding a pull-
up current greater than 5 μ A to the RUN/SS pin. The
additional current prevents the discharge of C SS during a
fault and also shortens the soft-start period. Using a
resistor to V IN as shown in Figure 6a is simple, but slightly
increases shutdown current. Connecting a resistor to
INTV CC as shown in Figure 6b eliminates the additional
shutdown current, but requires a diode to isolate C SS . Any
pull-up network must be able to pull RUN/SS above the
4.2V maximum threshold of the latchoff circuit and over-
come the 4 μ A maximum discharge current.
INTV CC Regulator
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3717-1. The INTV CC pin can supply up to
50mA RMS and must be bypassed to ground with a
minimum of 4.7 μ F tantalum or other low ESR capacitor.
Good bypassing is necessary to supply the high transient
currents required by the MOSFET gate drivers. Applica-
high frequency of operation may cause the LTC3717-1 to
exceed its maximum junction temperature rating or RMS
current rating. Most of the supply current drives the
MOSFET gates unless an external EXTV CC source is used.
In continuous mode operation, this current is I GATECHG =
f(Q g(TOP) + Q g(BOT) ). The junction temperature can be
estimated from the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3717EUH-1
is limited to less than 14mA from a 30V supply:
T J = 70 ° C + (14mA)(30V)(34 ° C/W) = 84.3 ° C
For larger currents, consider using an external supply with
the EXTV CC pin.
EXTV CC Connection
The EXTV CC pin can be used to provide MOSFET gate drive
and control power from the output or another external
source during normal operation. Whenever the EXTV CC
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTV CC
pin to INTV CC . INTV CC power is supplied from EXTV CC until
this pin drops below 4.5V. Do not apply more than 7V to
the EXTV CC pin and ensure that EXTV CC ≤ V CC . The follow-
ing list summarizes the possible connections for EXTV CC :
1. EXTV CC grounded. INTV CC is always powered from the
internal 5V regulator.
2. EXTV CC connected to an external supply. A high effi-
ciency supply compatible with the MOSFET gate drive
requirements (typically 5V) can improve overall
efficiency.
3. EXTV CC connected to an output derived boost network.
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The system
will start-up using the internal linear regulator until the
boosted output supply is available.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
tions using large MOSFETs with a high input voltage and
sn37171 37171fs
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