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| 型号： | LTC3717EUH-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 8/20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
OPERATIO
Main Control Loop
The LTC3717-1 is a current mode controller for DC/DC
step-down converters. In normal operation, the top
MOSFET is turned on for a fixed interval determined by a
one-shot timer OST. When the top MOSFET is turned off,
the bottom MOSFET is turned on until the current com-
parator I CMP trips, restarting the one-shot timer and initi-
ating the next cycle. Inductor current is determined by
sensing the voltage between the SENSE + and SENSE – pins
using the bottom MOSFET on-resistance . The voltage on
the I TH pin sets the comparator threshold corresponding
to inductor valley current. The error amplifier EA adjusts
this I TH voltage by comparing 2/3 of the feedback signal
V FB from the output voltage with a reference equal to 1/3
of the V REF voltage. If the load current increases, it causes
a drop in the feedback voltage relative to the reference. The
I TH voltage then rises until the average inductor current
again matches the load current. As a result in normal DDR
operation V OUT is equal to 1/2 of the V REF voltage.
The operating frequency is determined implicitly by the
top MOSFET on-time and the duty cycle required to
maintain regulation. The one-shot timer generates an on-
time that is proportional to the ideal duty cycle, thus
holding frequency approximately constant with changes
in V IN . The nominal frequency can be adjusted with an
external resistor R ON .
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback voltage
exits a ± 10% window around the regulation point.
Furthermore, in an overvoltage condition, M1 is turned off
and M2 is turned on and held on until the overvoltage
condition clears.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both M1 and M2. Releasing
the pin allows an internal 1.2 μ A current source to charge
up an external soft-start capacitor C SS . When this voltage
reaches 1.5V, the controller turns on and begins switch-
ing, but with the I TH voltage clamped at approximately
0.6V below the RUN/SS voltage. As C SS continues to
charge, the soft-start current limit is removed.
INTV CC /EXTV CC Power
Power for the top and bottom MOSFET drivers and most
of the internal controller circuitry is derived from the
INTV CC pin. The top MOSFET driver is powered from a
floating bootstrap capacitor C B . This capacitor is re-
charged from INTV CC through an external Schottky diode
D B when the top MOSFET is turned off. When the EXTV CC
pin is grounded, an internal 5V low dropout regulator
supplies the INTV CC power from V CC . If EXTV CC rises
above 4.7V, the internal regulator is turned off, and an
internal switch connects EXTV CC to INTV CC . This allows
a high efficiency source connected to EXTV CC , such as an
external 5V supply or a secondary output from the
converter, to provide the INTV CC power. Voltages up to
7V can be applied to EXTV CC for additional gate drive. If
the V CC voltage is low and INTV CC drops below 3.4V,
undervoltage lockout circuitry prevents the power
switches from turning on.
sn37171 37171fs
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