1.25A

Load Current

Increases

'I

Normal Operation

Current Limited

LM34910

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SNVS297A – OCTOBER 2004 – REVISED FEBRUARY 2005

Figure 7 illustrates the inductor current waveform. During normal operation the load current is Io, the average of

the ripple waveform. When the load resistance decreases the current ratchets up until the lower peak reaches

1.25A. During the Current Limited portion of Figure 7, the current ramps down to 1.25A during each off-time,

initiating the next on-time (assuming the voltage at FB is <2.5V). During each on-time the current ramps up an

amount equal to:

(7)

ΔI/2.

Figure 7. Inductor Current - Current Limit Operation

external resistor will typically be less than 1

N - Channel Buck Switch and Driver

The LM34910 integrates an N-Channel buck switch and associated floating high voltage gate driver. The peak

current allowed through the buck switch is 3.5A, and the maximum allowed average current is 3A. The gate

driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.022

µF capacitor (C4) connected between BST and SW provides the voltage to the driver during the on-time. During

minimum off-time of 280 ns ensures a minimum time each cycle to recharge the bootstrap capacitor.

Softstart

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

11.5 µA current source charges up the external capacitor at the SS pin to 2.5V. The ramping voltage at SS (and

the non-inverting input of the regulation comparator) ramps up the output voltage in a controlled manner.

Thermal Shutdown

The LM34910 should be operated so the junction temperature does not exceed 125°C. If the junction

temperature increases, an internal Thermal Shutdown circuit, which activates (typically) at 175°C, takes the

controller to a low power reset state by disabling the buck switch and the on-timer, and grounding the Softstart

pin. This feature helps prevent catastrophic failures from accidental device overheating. When the junction

temperature reduces below 155°C (typical hysteresis = 20°C), the Softstart pin is released and normal operation

resumes.

Copyright © 2004–2005, Texas Instruments Incorporated

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