NCP1422
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9
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 closed−loop operation regulates voltage at
pin1 (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 turned ON.
The previously mentioned situation occurs when the
regulator is operating in CCM, M2 is turned OFF, M1 is just
turned ON, and M2 is not completely turned OFF. A dead
time is also needed to make sure M2 is completely turned
OFF before M1 is 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.
Cycle−by−Cycle 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 sense−voltage is developed. The
threshold detector (I
LIM
) detects whether the
sense−voltage 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 OFF−time (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.
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.
True−Cutoff
The NCP1422 has a True−Cutoff function controlled by
the multi−function 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
reference voltage of the controller is disabled and the
controller typically consumes only 50 nA of current. If the
pin 2 voltage is raised to higher than 0.5 V (for example, by
a resistor connected to V
IN)
, the IC is enabled again, and the
internal circuit typically consumes 8.5 A of current from
the OUT pin during normal operation.
Low−Battery Detection
A comparator with 30 mV hysteresis is applied to
perform the low−battery detection function. When pin 2
(LBI/EN) is at a voltage (defined by a resistor divider from
the battery voltage) lower than the internal reference
voltage of 1.20 V, the comparator output turns on a 50
low side switch. It pulls down the voltage at pin 3 (LBO)
which has hundreds of k of pull−high resistance. If the
pin 2 voltage is higher than 1.20 V + 30 mV, the comparator
output turns off the 50 low side switch. When this occurs,
pin 3 becomes high impedance and its voltage is pulled
high again.
