LM34910CSDXSLASHNOPB datasheet(9/19 Pages) TI

9 / 19 page

1.25A

Load Current

Increases

IPK

IOCL

Normal Operation

Current Limited

LM34910C

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SNVS517B – MAY 2007 – REVISED MARCH 2013

Figure 6 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 6, 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:

ΔI = (VIN - VOUT) x tON / L1

(7)

During this time the LM34910C is in a constant current mode, with an average load current (IOCL) equal to 1.25A

ΔI/2.

Figure 6. Inductor Current - Current Limit Operation

The current limit threshold can be increased by connecting an external resistor between SGND and ISEN. The

external resistor will typically be less than 1

Ω. The peak current out of SW and ISEN must not exceed 3.5A. The

average current out of SW must be less than 3A, and the average current out of ISEN must be less than 2A.

Therefore IPK in Figure 6 must not exceed 3.5A, and IOCL must not exceed 2A.

N - Channel Buck Switch and Driver

The LM34910C 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

each off-time, the SW pin is at approximately -1V, and C4 charges from VCC through the internal diode. The

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

start-up stresses and current surges. Upon turn-on, after VCC reaches the under-voltage threshold, an internal

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.

An internal switch grounds the SS pin if VCC is below the under-voltage lockout threshold, if a thermal shutdown

occurs, or if the RON/SD pin is grounded.

Thermal Shutdown

The LM34910C 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.

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