LP5813 datasheet(14/60 Pages) TI

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9.3.1.2 Enable and Soft Start

When the input voltage is above the UVLO rising threshold 1.8 V and the EN pin is pulled to a voltage above

1.2 V, the boost of LP5813 is enabled and starts up. At the beginning, the LP5813 charges the boost output

capacitor with a constant current when the output voltage is below 0.4 V. When the output voltage is charged

above 0.4 V, the LP5813 has capability to drive 200 mA load. After the output voltage reaches the input voltage,

the boost of LP5813 starts switching, and the output voltage continually ramps up to default voltage 3 V. The

typical start-up time is 450 μs from EN pulled high to output reaching default voltage 3 V, at the application that

input voltage is 2.5 V, output effective capacitance is 10 μF, and no load. When the EN is below 0.42 V, the

internal enable comparator turns the device into shutdown mode. In the shutdown mode, the device is entirely

turned off and the output is disconnected from input power supply.

9.3.1.3 Switching Frequency

The LP5813 switches at quasi-constant 1-MHz frequency when the input voltage is above 1.5 V. When the input

voltage is lower than 1.5 V, the switching frequency is reduced gradually to 0.5 MHz, which improves the boost

efficiency and gets higher boost ratio. When the input voltage is below 1 V, the switching frequency is fixed at

quasi-constant 0.5 MHz.

9.3.1.4 Current Limit Operation

The LP5813 uses a valley current limit sensing scheme. The inductor current is detected during the switching

off-time, by sensing the voltage across the synchronous rectifier.

When the load current increases, such that the inductor current is above the current limit within the whole

switching cycle , the off-time increases to discharge the inductor current. The current decreases below the limit

before the next on-time. When the current limit is reached, the output voltage decreases if the load current

continually increases.

The maximum continuous output current (IOUT(CL)), before entering current limit (CL) operation, can be defined

by Equation 1.

IOUTCL = 1−D × ILIM+12∆ILP−P

(1)

where

• D is the duty cycle

• ΔIL(P-P) is the inductor ripple current

The duty cycle can be estimated by Equation 2.

D=1−VIN×ηVOUT

(2)

where

• VOUT is the output voltage of the boost converter

• VIN is the input voltage of the boost converter

• η is the efficiency of the converter, use 90% for most applications

The peak-to-peak inductor ripple current is calculated by Equation 3.

∆ILP−P =VIN×D

L×fSW

(3)

where

• L is the inductance value of the inductor

• fSW is the switching frequency

• D is the duty cycle

• VIN is the input voltage of the boost converter

LP5813

SNVSC74 – SEPTEMBER 2023

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