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參數資料
| 型號： | NCP1421D |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | DC/DC變換器 |
| 英文描述： | 600 mA Sync-Rect PFM Step-Up DC-DC Converter Step-Up DC-DC Converter with True-Cutoff and Ring-Killer |
| 中文描述： | 600毫安同步整流PFM升壓型DC - DC轉換器升壓型DC - DC轉換器與True - Cutoff和Ring殺手 |
| 文件頁數： | 9/14頁 |
| 文件大小： | 116K |
| 代理商： | NCP1421D |
NCP1421
http://onsemi.com
9
DETAILED OPERATION DESCRIPTION
NCP1421 is a monolithic micropower highfrequency
stepup voltage switching converter IC specially designed
for battery operated handheld electronic products up to
600 mA loading. It integrates a Synchronous Rectifier to
improve efficiency as well as to eliminate the external
Schottky diode. High switching frequency (up to 1.2 MHz)
allows for a low profile inductor and output capacitor to be
used. LowBattery Detector, LogicControlled Shutdown,
and CyclebyCycle Current Limit provide valueadded
features for various batteryoperated applications. With all
these functions ON, the quiescent supply current is
typically only 8.5 A. This device is available in a compact
Micro8 package.
PFM Regulation Scheme
From the simplified functional diagram (Figure 1), the
output voltage is divided down and fed back to pin 1 (FB).
This voltage goes to the noninverting input of the PFM
comparator whereas the comparator’s inverting input is
connected to the internal voltage reference, REF. A
switching cycle is initiated by the falling edge of the
comparator, at the moment the main switch (M1) is turned
ON. After the maximum ONtime (typically 0.72 S)
elapses or the current limit is reached, M1 is turned OFF
and the synchronous switch (M2) is turned ON. The M1
OFF time is not less than the minimum OFFtime
(typically 0.12
S), which ensures complete energy
transfer from the inductor to the output capacitor. If the
regulator is operating in Continuous Conduction Mode
(CCM), M2 is turned OFF just before M1 is supposed to be
ON again. If the regulator is operating in Discontinuous
Conduction Mode (DCM), which means the coil current
will decrease to zero before the new cycle starts, M1 is
turned OFF as the coil current is almost reaching zero. The
comparator (ZLC) with fixed offset is dedicated to sense
the voltage drop across M2 as it is conducting; when the
voltage drop is below the offset, the ZLC comparator
output goes HIGH and M2 is turned OFF. Negative
feedback of closedloop operation regulates voltage at
pin 1 (FB) equal to the internal reference voltage (1.20 V).
Synchronous Rectification
The Synchronous Rectifier is used to replace the
Schottky Diode to reduce the conduction loss contributed
by the forward voltage of the Schottky Diode. The
Synchronous Rectifier is normally realized by powerFET
with gate control circuitry that incorporates relatively
complicated timing concerns.
As the main switch (M1) is being turned OFF and the
synchronous switch M2 is just turned ON with M1 not
being completely turned OFF, current is shunt from the
output bulk capacitor through M2 and M1 to ground. This
power loss lowers overall efficiency and possibly damages
the switching FETs. As a general practice, a certain amount
of dead time is introduced to make sure M1 is completely
turned OFF before M2 is being turned ON.
The previously mentioned situation occurs when the
regulator is operating in CCM, M2 is being turned OFF, M1
is just turned ON, and M2 is not being completely turned
OFF. A dead time is also needed to make sure M2 is
completely turned OFF before M1 is being turned ON.
As coil current is dropped to zero when the regulator is
operating in DCM, M2 should be OFF. If this does not
occur, the reverse current flows from the output bulk
capacitor through M2 and the inductor to the battery input,
causing damage to the battery. The ZLC comparator comes
with fixed offset voltage to switch M2 OFF before any
reverse current builds up. However, if M2 is switched OFF
too early, large residue coil current flows through the body
diode of M2 and increases conduction loss. Therefore,
determination of the offset voltage is essential for optimum
performance. With the implementation of the synchronous
rectification scheme, efficiency can be as high as 94% with
this device.
CyclebyCycle Current Limit
In Figure 1, a SENSEFET is used to sample the coil
current as M1 is ON. With that sample current flowing
through a sense resistor, a sensevoltage is developed. The
threshold
detector
(I
LIM
)
sensevoltage is higher than the preset level. If the sense
voltage is higher than the present level, the detector output
notifies the Control Logic to switch OFF M1, and M1 can
only be switched ON when the next cycle starts after the
minimum OFFtime (typically 0.12 S). With proper
sizing of the SENSEFET and sense resistor, the peak coil
current limit is typically set at 1.5 A.
detects
whether
the
Voltage Reference
The voltage at REF is typically set at 1.20 V and can
output up to 2.5 mA with load regulation
±
2% at V
OUT
equal to 3.3 V. If V
OUT
is increased, the REF load
capability can also be increased. A bypass capacitor of
200 nF is required for proper operation when REF is not
loaded. If REF is loaded, a 1.0 F capacitor at the REF pin
is needed.
TrueCutoff
The NCP1421 has a TrueCutoff function controlled by
the multifunction pin LBI/EN (pin 2). Internal circuitry
can isolate the current through the body diode of switch M2
to load. Thus, it can eliminate leakage current from the
battery to load in shutdown mode and significantly reduce
battery current consumption during shutdown. The
shutdown function is controlled by the voltage at pin 2
(LBI/EN). When pin 2 is pulled to lower than 0.3 V, the
controller enters shutdown mode. In shutdown mode, when
switches M1 and M2 are both switched OFF, the internal
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