Voltage

at CS

0V

VRIPPLE

tOFF

tON

Inductor

Current

'I

13

LM25011, LM25011-Q1, LM25011A, LM25011A-Q1

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SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014

Product Folder Links: LM25011 LM25011-Q1 LM25011A LM25011A-Q1

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Feature Description (continued)

7.3.4 Ripple Requirements

The LM25011 requires about 25 mVP-P of ripple voltage at the CS pin. Higher switching frequencies may require

more ripple. That ripple voltage is generated by the decreasing recirculating current (the inductor ripple current)

through RS during the off-time. See Figure 16.

Figure 16. CS Pin Waveform

The ripple voltage is equal to:

VRIPPLE = ΔI × RS

(7)

where ΔI is the inductor current ripple amplitude, and RS is the current-sense resistor at the CS pin.

More ripple can be achieved by decreasing the inductor value.

The LM25011A, with its shorter minimum off-time, typically will require more ripple than the LM25011. An

external circuit to increase the effective ripple voltage may be needed. Different methods of generating this ripple

are explained in the External Components section.

7.3.5 N-Channel Buck Switch and Driver

The LM25011 integrates an N-Channel buck switch and associated floating high-voltage gate driver. The gate

driver circuit works in conjunction with an external bootstrap capacitor (CBST) and an internal high-voltage diode.

A 0.1-µF capacitor connected between BST and SW provides the supply voltage for the driver during the on-

time. During each off-time, the SW pin is at approximately –1 V, and CBST is recharged from the internal 5-V

regulator for the next on-time. The minimum off-time ensures a sufficient time each cycle to recharge the

bootstrap capacitor.

In applications with relatively high output voltage and low minimum load current, the internal pre-charge device of

the LM25011 may not pull the SW pin sufficiently low during the off-time to maintain the voltage on the bootstrap

capacitor. If the bootstrap capacitor (CBST) discharges during the long off-times, and the regulator will cycle on

and off at a low frequency. Decreasing the values of the feedback resistors RFB1 and RFB2 to provide a minimum

load of typically 1mA at nominal VOUT will increase the minimum switching frequency and maintain sufficient

bootstrap capacitor voltage.

7.3.6 Soft-Start

The soft-start feature allows the converter to gradually reach a steady-state operating point, thereby reducing

startup stresses and current surges. Upon turn-on, when VIN reaches its undervoltage lock-out threshold an

internal 10-µA current source charges the external capacitor at the SS pin to 2.51 V (t1 in Figure 17). The

ramping voltage at SS ramps the non-inverting input of the regulation comparator and the output voltage, in a

controlled manner. For proper operation, the soft-start capacitor should be no smaller than 1000 pF.