LM27341_14 datasheet(12/42 Pages) TI1

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LM27342

VIN

PVIN

BOOST

GND/DAP

VOUT

SYNC

AVIN

CLK

OFF

LM27341, LM27342, LM27341Q, LM27342Q

SNVS497D – NOVEMBER 2008 – REVISED FEBRUARY 2010

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Figure 33. External Diode Charges CBOOST

HIGH OUTPUT VOLTAGE CONSIDERATIONS

When the output voltage is greater than 3.3V, a minimum load current is needed to charge CBOOST, see

Figure 34. The minimum load current forward biases the catch diode D1 forcing the SW pin to swing below

ground. This allows CBOOST to charge, ensuring that the gate drive voltage is high enough for proper operation.

The minimum load current depends on many factors including the inductor value.

Figure 34. Minimum Load Current for L = 1.5 µH

ENABLE PIN / SHUTDOWN MODE

Connect the EN pin to a voltage source greater than 1.8V to enable operation of the LM27341/LM27342. Apply a

voltage less than 0.4V to put the part into shutdown mode. In shutdown mode the quiescent current drops to

typically 70 nA. Switch leakage adds another 40 nA from the input supply. For proper operation, the

LM27341/LM27342 EN pin should never be left floating, and the voltage should never exceed VIN + 0.3V.

The simplest way to enable the operation of the LM27341/LM27342 is to connect the EN pin to AVIN which

allows self start-up of the LM27341/LM27342 when the input voltage is applied.

When the rise time of VIN is longer than the soft-start time of the LM27341/LM27342 this method may result in an

overshoot in output voltage. In such applications, the EN pin voltage can be controlled by a separate logic signal,

or tied to a resistor divider, which reaches 1.8V after VIN is fully established (see Figure 35). This will minimize

the potential for output voltage overshoot during a slow VIN ramp condition. Use the lowest value of VIN , seen in

your application when calculating the resistor network, to ensure that the 1.8V minimum EN threshold is reached.

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