2

DC =

1.18 x 10

-10

9

LM25010, LM25010-Q1

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SNVS419E – DECEMBER 2005 – REVISED MAY 2016

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Feature Description (continued)

The LM25010 requires a minimum of 25-mV of ripple voltage at the FB pin for stable fixed-frequency operation. If

the output capacitor’s ESR is insufficient, additional series resistance may be required (R3 in the Functional

Block Diagram).

The LM25010 operates in continuous conduction mode at heavy load currents, and discontinuous conduction

mode at light load currents. In continuous conduction mode current always flows through the inductor, never

decaying to zero during the OFF-time. In this mode the operating frequency remains relatively constant with load

and line variations. The minimum load current for continuous conduction mode is one-half the inductor’s ripple

current amplitude. Calculate the operating frequency in the continuous conduction mode with Equation 2.

(2)

The buck switch duty cycle is equal to Equation 3.

(3)

Under light load conditions, the LM25010 operates in discontinuous conduction mode, with zero current flowing

through the inductor for a portion of the OFF-time. The operating frequency is always lower than that of the

continuous conduction mode, and the switching frequency varies with load current. Conversion efficiency is

maintained at a relatively high level at light loads because the switching losses diminish as the power delivered

to the load is reduced. Calculate the approximate discontinuous mode operating frequency with Equation 4.

where

•

(4)

A high voltage bias regulator is integrated within the LM25010. The input pin (VIN) can be connected directly to

capacitor at VCC (C3) must be a minimum of 0.47 µF to prevent the voltage at VCC from rising above its

absolute maximum rating in response to a step input applied at VIN. C3 must be located as close as possible to

the LM25010 pins.

In applications with a relatively high input voltage, power dissipation in the bias regulator is a concern. An

auxiliary voltage of between 7.5 V and 14 V can be diode connected to the VCC pin (D2 in Figure 8) to shut off

the VCC regulator, reducing internal power dissipation. The current required into the VCC pin is shown in the

Typical Performance Characteristics. Internally a diode connects VCC to VIN requiring that the auxiliary voltage

The turn-on sequence is shown in Figure 1. When VCC exceeds the undervoltage lockout threshold (UVLO) of

5.25 V (t1 in Figure 1), the buck switch is enabled, and the SS pin is released to allow the softstart capacitor (C6)

softstart voltage increases (t2 in Figure 1).