NCP1411
http://onsemi.com
9
DETAILED OPERATION DESCRIPTIONS
NCP1411 is a monolithic micropower high frequency
step−up voltage switching converter IC specially designed
for battery operated hand−held electronic products up to
250 mA loading. It integrates Synchronous Rectifier for
improving efficiency as well as eliminating the external
Schottky Diode. High switching frequency (up to 600 kHz)
allows low profile inductor and output capacitor being used.
Low−Battery Detector, Logic−Controlled Shutdown and
Cycle−by−Cycle Current Limit provide value−added
features for various battery−operated application. With all
these functions ON, the quiescent supply current is only
9.0 mA typical. This device is available in a compact Micro8
package.
PFM Regulation Scheme
From the simplified Functional Diagram (Figure 2), the
output voltage is divided down and fed back to pin 1 (FB).
This voltage goes to the non−inverting input of the PFM
comparator whereas the comparator’s inverting input is
connected to 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 ON−time
(typical 1.4 mS) 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
OFF−time (typical 0.31 mS), this is to ensure energy transfer
from the inductor to the output capacitor. If the regulator is
operating at Continuous Conduction Mode (CCM), M2 is
turned OFF just before M1 is supposed to be ON again. If the
regulator is operating at Discontinuous Conduction Mode
(DCM), which means the coil current will decrease to zero
before the next cycle, 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 pin 1 (FB) equal to the internal voltage reference
(1.190 V).
Synchronous Rectification
Synchronous Rectifier is used to replace Schottky Diode
for eliminating the conduction loss contributed by forward
voltage of the latter. Synchronous Rectifier is normally
realized by powerFET with gate control circuitry which,
however, involved relative complicated timing concerns.
As main switch M1 is being turned OFF, if the
synchronous switch M2 is just turned ON with M1 not being
completed turned OFF, current will be shunt from the output
bulk capacitor through M2 and M1 to ground. This power
loss lowers overall efficiency. So a certain amount of dead
time is introduced to make sure M1 is completely OFF
before M2 is being turned ON.
When the main regulator is operating in CCM, as M2 is
being turned OFF, and M1 is just turned ON with M2 not
being completely turned OFF, the above mentioned
situation will occur. So dead time is introduced to make sure
M2 is completely turned OFF before M1 is being turned ON.
When the regulator is operating in DCM, as coil current
is dropped to zero, M2 is supposed to be OFF. Fail to do so,
reverse current will flow from the output bulk capacitor
through M2 and then the inductor to the battery input. It
causes damage to the battery. So the ZLC comparator comes
with fixed offset voltage to switch M2 OFF before any
reverse current builds up. However, if M2 is switch OFF too
early, large residue coil current flows through the body diode
of M2 and increases conduction loss. Therefore,
determination on the offset voltage is essential for optimum
performance.
With the implementation of synchronous rectification,
efficiency can be as high as 92%. For single cell input
voltage, use an external Schottky diode such as MBR0520
connected from pin 7 to pin 8 to ensure quick startup.
Ring−Killer
When the device entered Discontinuous Conduction
Mode operation, a typical ringing at LX pin will start while
the inductor current just ceased. This ringing is caused
primarily by the capacitance and inductance at LX node and
the result can produce unwanted EMI problem to the system.
In order to eliminate this ringing, an internal damping switch
(M3) is implemented to provide a low impedance path to
dissipate the residue energy stored in the inductor once the
operation entered the Discontinuous Conduction Mode.
This feature can improve the EMI problem. The
performance of the Ring−Killer switch is shown in
Figure 22.
Cycle−by−Cycle Current Limit
From Figure 2, SENSEFET is applied to sample the coil
current as M1 is ON. With that sample current flowing
through a sense resistor, sense−voltage is developed.
Threshold detector (ILIM) detects whether the
sense−voltage is higher than preset level. If it happens,
detector output signifies the CONTROL LOGIC to switch
OFF M1, and M1 can only be switched ON as next cycle
starts after the minimum OFF−time (typical 0.31 mS). With
properly sizing of SENSEFET and sense resistor, the peak
coil current limit is set at 1.0 A typically.
Voltage Reference
The voltage at REF is set typically at +1.190 V. It can
deliver up to 2.5 mA with load regulation ±1.5%, 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 0.15 mF is
required for proper operation when REF is not loaded. If
REF is loaded, 1.0 mF capacitor at REF is needed.
Shutdown
The IC will shutdown when the voltage at pin 2 (LBI/EN)
is pulled lower than 0.3 V. During shutdown, M1 and M2 are
both switched OFF, however, the body diode of M2 allows
current flow from battery to the output, the IC internal circuit
will consume less than 0.05 mA current typically. If the
