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參數資料
| 型號： | NCP1508 |
| 廠商： | ON SEMICONDUCTOR |
| 英文描述： | Up to 500 mA, 1 MHz, High Efficiency Synchronous Step−Down DC−DC Converter in DFN Package |
| 文件頁數： | 10/15頁 |
| 文件大小： | 414K |
| 代理商： | NCP1508 |
NCP1508
http://onsemi.com
10
DETAILED OPERATING DESCRIPTION
Overview
The NCP1508 is a monolithic micro
power high
frequency PWM step
down DC
DC converter specifically
optimized for applications requiring high efficiency and a
small PCB footprint such as portable battery powered
products. It integrates synchronous rectification to
improve efficiency as well as eliminate the external
Schottky diode. High switching frequency allows for a low
profile inductor and capacitors to be used. Four digital
selectable output voltages (1.0, 1.3, 1.5 and 1.89 V) can be
generated from the input supply that can range from
2.7
5.2 V. All loop compensation is integrated as well
further reducing the external component count as well.
The DC
DC converter has two operating modes (normal
PWM, pulsed switching), which are intended to allow for
optimum efficiency under either light (up to 30 mA) or
heavy loads. The user determines the operating mode by
controlling the SYNC input. In addition the SYNC input
can be used to synchronize the PWM to an external system
clock signal in the range of 500
1000 kHz.
PWM Operating Mode
The NCP1508 can be set to current mode PWM
operation by connecting SYNC pin to V
CC
. In this mode,
the output voltage is regulated by modulating on
time
pulse width of the main switch Q1 at a fixed frequency of
1 MHz. The switching of the PMOS Q1 is controlled by a
flip
flop driven by the internal oscillator and a comparator
that compares the error signal from an error amplifier with
the sum of the sensed current signal and compensation
ramp. At the beginning of each cycle, the main switch Q1
is turned ON by the rising edge of the internal oscillation
clock. The inductor current ramps up until the sum of the
current sense signal and compensation ramp becomes
higher than the error voltage amplifier. Once this has
occurred, the PWM comparator resets the flip
flop, Q1 is
turned OFF and the synchronous switch Q2 is turned ON.
Q2 replaces the external Schottky diode to reduce the
conduction loss and improve the efficiency. To avoid
overall power loss, a certain amount of dead time is
introduced to ensure Q1 is completely turned OFF before
Q2 is being turned ON.
In continuous conduction mode (CCM), Q1 is turned ON
after Q2 is completely turned OFF to start a new clock
cycle. In discontinuous conduction mode (DCM), the zero
crossing comparator (ZLC) will turn off Q2 when the
inductor current drops to zero.
Overvoltage Protection
The overvoltage protection circuit is present in PWM
mode to prevent the output voltage from going too high
under light load or fast load transient conditions. The
output overvoltage threshold is 5% above nominal set
value. If the output voltage rises above 5% of the nominal
value, the OVP comparator is activated and switch Q1 is
turned OFF. Switching will continue when the output
voltage falls below the threshold of OVP comparator.
Pulsed Mode (PM)
Under light load conditions (< 30 mA), NCP1508 can be
configured to enter a low current pulsed mode operation to
reduce power consumption. This is accomplished by
SYNC pin held LOW. The output regulation is
implemented by pulse frequency modulation. If the output
voltage drops below the threshold of PM comparator
(typically Vnom
2%), a new cycle will be initiated by the
PM comparator to turn on the switch Q1. Q1 remains ON
until 200 mA inductor peak current is reached. Then ILIM
comparator goes high to switch off Q1. After a short dead
time delay, switch rectifier Q2 is turn ON. The zero
crossing comparator will detect when the inductor current
drops to zero and send the signal to turn off Q2. The output
voltage continues to decrease through discharging the
output capacitor. When the output voltage falls below the
threshold of PM comparator again, a new cycle starts
immediately.
Cycle
by
Cycle Current Limit
From the block diagram (Figure 3), an ILIM comparator
is used to realize cycle
by
cycle current limit protection.
The comparator compares the LX pin voltages with the
reference voltage from the SENFET, which is biased by
constant current. If the inductor current reaches the limit,
ILIM comparator detects the LX voltage falling below the
reference voltage from SENFET and releases the signal to
turn off the switch Q1. The cycle
by
cycle current limit is
set at 800 mA in PWM and 200 mA in PM.
Frequency Synchronization and Operating Mode
Selection
The SYNC pin can also be used for frequency
synchronization by connecting it with an external clock
signal. It operates in PWM mode when synchronized to an
external clock. The switching cycle initiates by the rising
edge of the clock. The synchronization clock signals
between 0.4 V and 1.2 V from 500 kHz to 1000 kHz.
Gating on and off the clock, the SYNC pin can also be
used to select between PM and PWM modes. It allows
efficient dynamical power management by adjusting the
converter operation to the specific system requirement. Set
SYNC pin low to select PM mode at light load conditions
(up to 30 mA) and set SYNC pin high or connect with
external clock to select PWM mode at heavy load condition
to achieve optimum efficiency. Table 1 shows the mode
selection with three different SYNC pin states.
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相關代理商/技術參數 |
參數描述 |
|---|---|
| NCP1508MNR2 | 功能描述:直流/直流開關轉換器 High Efficiency Buck RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數量:2 最大工作溫度:+ 85 C 安裝風格:SMD/SMT |
| NCP1508MNR2G | 功能描述:直流/直流開關轉換器 High Efficiency Buck Synchronous DC/DC RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數量:2 最大工作溫度:+ 85 C 安裝風格:SMD/SMT |
| NCP1509MNR2 | 功能描述:直流/直流開關轉換器 High Efficiency Buck RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數量:2 最大工作溫度:+ 85 C 安裝風格:SMD/SMT |
| NCP1509MNR2G | 功能描述:直流/直流開關轉換器 High Efficiency Buck Synchronous DC/DC RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數量:2 最大工作溫度:+ 85 C 安裝風格:SMD/SMT |
| NCP1510AFCT1 | 功能描述:直流/直流開關轉換器 High Efficiency Buck RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數量:2 最大工作溫度:+ 85 C 安裝風格:SMD/SMT |
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