LM2773

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SNVS554A – JANUARY 2008 – REVISED MAY 2013

OPERATION DESCRIPTION

Overview

The LM2773 is a switched capacitor converter that produces a selectable 1.8V or 1.6V regulated output. The

core of the part is a highly efficient charge pump that utilizes fixed frequency pre-regulation, Pulse Frequency

Modulation, and spread spectrum to minimize conducted noise and power losses over wide input voltage and

output current ranges. A description of the principal operational characteristics of the LM2773 is detailed in the

Circuit Description, and Efficiency Performance sections. These sections refer to details in the Block Diagram.

Circuit Description

The core of the LM2773 is a two-phase charge pump controlled by an internally generated non-overlapping

clock. The charge pump operates by using external flying capacitors C1 and C2 to transfer charge from the input

to the output. The LM2773 will operate in a 1x Gain, with the input current being equal to the load current, when

the input voltage is at or below 3.5V (typ.) for 1.8V mode or 3.3V (typ.) for 1.6V mode. At input voltages above

3.5V (typ.) or 3.3V (typ.) for the respective voltage mode selected, the part utilizes a gain of 2/3x, resulting in an

input current equal to 2/3 times the load current.

The two phases of the switched capacitor switching cycle will be referred to as the "charge phase" and the

"discharge phase". During the charge phase, the flying capacitor is charged by the input supply. After half of the

switching cycle [ t = 1/(2×FSW) ], the LM2773 switches to the discharge phase. In this configuration, the charge

that was stored on the flying capacitors in the charge phase is transferred to the output.

The LM2773 uses fixed frequency pre-regulation to regulate the output voltage to 1.8V during moderate to high

load currents. The input and output connections of the flying capacitors are made with internal MOS switches.

Pre-regulation limits the gate drive of the MOS switch connected between the voltage input and the flying

capacitors. Controlling the on resistance of this switch limits the amount of charge transferred into and out of

each flying capacitor during the charge and discharge phases, and in turn helps to keep the output ripple very

low.

When output currents are low (<40mA typ.), the LM2773 automatically switches to a low-ripple Pulse Frequency

Modulation (PFM) form of regulation. In PFM mode, the flying capacitors stay in the discharge phase until the

output voltage drops below a predetermined trip point. When this occurs, the flying capacitors switch back to the

charge phase. After being charged, the flying capacitors repeat the process of staying in the discharge phase

and switching to the charge phase when necessary.

The LM2773 utilizes spread spectrum operation to distrubute the peak radiated energy of the device over a wider

frequency band, reducing electromagnetic interference (EMI). Spread spectrum is used during all modes of

operation for the LM2773.

Efficiency Performance

Charge-pump efficiency is derived in the following two ideal equations (supply current and other losses are

neglected for simplicity):

IIN = G × IOUT

(1)

E = (VOUT × IOUT) ÷ (VIN × IIN) = VOUT ÷ (G × VIN)

(2)

In the equations, G represents the charge pump gain. Efficiency is at its highest as G×VIN approaches VOUT.

Refer to the efficiency graph in the Typical Performance Characteristics section for detailed efficiency data. The

transition between the gain of 1x and 2/3x is clearly distinguished by the sharp discontinuity in the efficiency

curve.

Shutdown and Voltage Select

The LM2773 is in shutdown mode when the voltage on the enable pin (EN) is logic-low. In shutdown, the

LM2773 draws virtually no supply current. When in shutdown, the output of the LM2773 is completely

disconnected from the input. Internal feedback resistors pull the output voltage down to 0V during shutdown.

The SEL pin sets the output voltage at either 1.8V or 1.6V. A logic-low voltage on the SEL pin will place the

output of the LM2773 in the 1.6V mode, and a logic-high voltage on the SEL pin will place it into the 1.8V mode.

There are 350k

Ω pull-down resistors connected internally between the EN pin and GND and the SEL pin and

GND.

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Product Folder Links: LM2773