Preliminary Datasheet

LP6252

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Page 5 of 8

Operation Information

The LP6252 uses a synchronous 550KHz fixed

frequency, current-mode regulation architecture to

regulate the output voltage. The LP6252 measures the

output voltage through an external resistive voltage

divider and compares that to the internal 0.8V

reference to generate the error voltage to the inductor

current to regulate the output voltage. The use of

current-mode regulation improves transient response

and control loop stability.

When the LP6252 is disable (EN=Low), both power

switches are off. There is no current path from SW to

OUT. Therefore, the output voltage discharges to

ground. When the LP6252 is enabled(EN=High),a

limited start-current charges the output voltage rising

to SW, then the part operates in force PWM mode for

regulating the output voltage to the target value. At the

beginning of each cycle, the N-channel MOSFET

switch is turned on, forcing the inductor current to rise,

The current at the source of the switch is internally

measured and converted to a voltage by the current

sense amplifier. That voltage is compared to the error

voltage. When the inductor current rises sufficiently,

the PWM comparator turns off the switch, forcing the

inductor current to the output capacitor through the

internal P-Channel MOSFET rectifier, which forces the

inductor current to decrease. The peak inductor

current is controlled by the error voltage. Thus the

output voltage controls the inductor current to satisfy

the lode.

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. The voltage divider drops the

output voltage to the 0.8V feedback voltage. Use a

100K resistor for R2 of the voltage divider. Determine

the high-side resistor R1 by the equation:

Vout=(R1/R2+1) x VFB

Pre-Boost Current and Short Circuit Protect

Initially output voltage is lower than battery voltage,

and the LP6252 enters pre-boost phase. During

pre-boost phase, the internal NMOSFET/PMOSFET is

turned off/on and a constant current is provided from

battery to output until the output voltage close to the

battery voltage. The constant current is limited by

internal controller. If the output short to ground, the

LP6252 also limits the output current to avoid damage

condition

Inductor Selection

For a better efficiency in high switching frequency

converter, the inductor selection has to use a proper

core material such as ferrite core to reduce the core

loss and choose low ESR wire to reduce copper loss.

The most important point is to prevent the core

saturated when handling the maximum peak current.

Using a shielded inductor can minimize radiated noise

in sensitive applications. The maximum peak inductor

current is the maximum input current plus the half of

inductor ripple current. The calculated peak current

has to be smaller than the current limitation in the

electrical characteristics. A typical setting of the

inductor ripple current is 20% to 40% of the maximum

input current. If the selection is 40%, the maximum

peak inductor current is