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NCP1411DMR2

Sync-Rect PFM Step-Up DC-DC Converter with Low-Battery Detector and Ring-Killer

ON Semiconductor 

NCP1411

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 VOUT

equal to 3.3 V. If VOUT 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

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