LM46002PWPR datasheet(23/52 Pages) TI

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LM46002

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SNVSA13B – APRIL 2014 – REVISED SEPTEMBER 2014

8.4 Device Functional Modes

8.4.1 Shutdown Mode

The EN pin provides electrical ON and OFF control for the LM46002. When VEN is below 0.4 V, the device is in

shutdown mode. Both the internal LDO and the switching regulator are off. In shutdown mode the quiescent

current drops to 2.3 µA typically with VIN = 24 V. The LM46002 also employs under voltage lock out protection. If

VCC voltage is below the UVLO level, the output of the regulator will be turned off.

8.4.2 Stand-by Mode

The internal LDO has a lower enable threshold than the regulator. When VEN is above 1.2 V and below the

precision enable falling threshold (1.8 V typically), the internal LDO regulates the VCC voltage at 3.2 V. The

precision enable circuitry is turned on once VCC is above the UVLO threshold. The switching action and voltage

regulation are not enabled unless VEN rises above the precision enable threshold (2.1 V typically).

8.4.3 Active Mode

The LM46002 is in Active Mode when VEN is above the precision enable threshold and VCC is above its UVLO

level. The simplest way to enable the LM46002 is to connect the EN pin to VIN. This allows self start-up of the

LM46002 when the input voltage is in the operation range: 3.5 V to 60 V. Please refer to Enable (ENABLE) and

VCC, UVLO and BIAS for details on setting these operating levels.

In Active Mode, depending on the load current, the LM46002 will be in one of four modes:

1. Continuous conduction mode (CCM) with fixed switching frequency when load current is above half of the

peak-to-peak inductor current ripple;

2. Discontinuous conduction mode (DCM) with fixed switching frequency when load current is lower than half of

the peak-to-peak inductor current ripple in CCM operation;

3. Pulse Frequency Modulation (PFM) when switching frequency is decreased at very light load;

4. Fold-back mode when switching frequency is decreased to maintain output regulation at lower supply voltage

VIN.

8.4.4 CCM Mode

Continuous Conduction Mode (CCM) operation is employed in the LM46002 when the load current is higher than

half of the peak-to-peak inductor current. In CCM operation, the frequency of operation is fixed unless the

minimum HS switch ON-time (TON_MIN), the mininum HS switch OFF-time (TOFF_MIN) or LS current limit is

exceeded. Output voltage ripple will be at a minimum in this mode and the maximum output current of 2 A can

be supplied by the LM46002.

8.4.5 Light Load Operation

When the load current is lower than half of the peak-to-peak inductor current in CCM, the LM46002 will operate

in Discontinuous Conduction Mode (DCM), also known as Diode Emulation Mode (DEM). In DCM operation, the

LS FET is turned off when the inductor current drops to 0 A to improve efficiency. Both switching losses and

conduction losses are reduced in DCM, comparing to forced PWM operation at light load.

At even lighter current loads, Pulse Frequency Mode (PFM) is activated to maintain high efficiency operation.

When the HS switch ON-time reduces to TON_MIN or peak inductor current reduces to its minimum IPEAK-MIN, the

switching frequency will reduce to maintain proper regulation. Efficiency is greatly improved by reducing

switching and gate drive losses.

8.4.6 Self-Bias Mode

For highest efficiency of operation, it is recommended that the BIAS pin be connected directly to VOUT when 3.3

≤ VOUT ≤ 28 V. In this Self-Bias Mode of operation, the difference between the input and output voltages of the

internal LDO are reduced and therefore the total efficiency of the LM46002 is improved. These efficiency gains

are more evident during light load operation. During this mode of operation, the LM46002 operates with a

minimum quiescent current of 27 µA (typical). Please refer to VCC, UVLO and BIAS for more details.

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