LM2623

SNVS188H – MAY 2004 – REVISED NOVEMBER 2014

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9 Power Supply Recommendations

The LM2623 can start up from voltages as low as 1.1 V. On start-up, the control circuitry switches the N-channel

MOSFET continuously until the output reaches 3 V. After this output voltage is reached, the normal step-up

regulator feedback and gated oscillator control scheme take over. Once the device is in regulation, it can operate

down to below 0.8 V input, since the internal power for the IC can be boot-strapped from the output using the

10 Layout

10.1 Layout Guidelines

The example layouts below follow proper layout guidelines and should be used as a guide for laying out the

LM2623 circuit. The LM2623 inductive boost converter sees a high switched voltage at the SW pin, and a step

current through the Schottky diode and output capacitor each switching cycle. The high switching voltage can

create interference into nearby nodes due to electric field coupling (I = C x dV/dt). The large step current through

the diode and the output capacitor can cause a large voltage spike at the SW and BOOST pins due to parasitic

inductance in the step current conducting path (V = L x di/dt). Board layout guidelines are geared towards

minimizing this electric field coupling and conducted noise.

Boost Output Capacitor Placement, Schottky Diode Placement, and Boost Input / VDD Capacitor Placement detail

the main (layout sensitive) areas of the LM2623 inductive boost converter in order of decreasing importance:

10.1.1 Boost Output Capacitor Placement

Because the output capacitor is in the path of the inductor current discharge path, it will see a high-current step

from 0 to IPEAK each time the switch turns off and the Schottky diode turns on. Any inductance along this series

path from the diodes cathode, through COUT, and back into the LM2623 GND pin will contribute to voltage

spikes at SW. These spikes can potentially over-voltage the SW and BOOST pins, or feed through to GND. To

avoid this, COUT+ must be connected as close as possible to the cathode of the Schottky diode, and COUT

−

must be connected as close as possible to the LM2623 GND bumps. The best placement for COUT is on the

same layer as the LM2623 to avoid any vias that can add excessive series inductance.

10.1.2 Schottky Diode Placement

In the LM2623 device boost circuit the Schottky diode is in the path of the inductor current discharge. As a result

the Schottky diode sees a high-current step from 0 to IPEAK each time the switch turns off, and the diode turns

on. Any inductance in series with the diode will cause a voltage spike at SW. This can potentially over-voltage

the SW pin, or feed through to VOUT and through the output capacitor, into GND. Connecting the anode of the

diode as close as possible to the SW pin, and connecting the cathode of the diode as close as possible to

COUT+, will reduce the inductance (LP_) and minimize these voltage spikes.

The LM2623 input capacitor filters the inductor current ripple and the internal MOSFET driver currents. The

inductor current ripple can add input voltage ripple due to any series resistance in the input power path. The

on the VIN/VDD supply line and GND.

12

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