White LED Charge Pump Board Layout Guidelines

　　For white LED drivers or high-current charge pumps with a large number of pins, there are some things to pay attention to when designing a printed circuit board (PCB). This article discusses the relevant design guidelines and layout rules using the MAX1576 as an example.
　　
　　For the MAX1576 white LED charge pump, the following rules need to be followed in the circuit board layout:

　　All GND and PGND pins are directly connected to the exposed pad (EP) under the IC.

　　It is best to use ceramic capacitors with X5R electrolyte or better for input, output and flying capacitors. Low ESR is critical for high current output, low input/output ripple and stability.

　　To avoid switching noise from the IC bias circuit, it is necessary to place the input capacitors (CIN and CINP) as close to the input and ground pins as possible, preferably without vias between the capacitors and the IC.

　　

　　If there are separate GND and PGND and/or IN and PIN pins, the IC contains separate power and bias inputs. If these pins are not close together, two input capacitors are required: CINP from PIN to PGND and CIN from IN to GND, each of which should be placed as close to the IC as possible. In this case, PIN and PGND should be connected to the system power and ground planes, respectively, while IN and GND are connected nearby. The PCB power traces first enter CINP and INP, and then connect to CIN and IN through some vias, providing some input noise filtering at the IN end. PGND and GND should be connected together through an exposed pad.

　　To ensure stable operation of the charge pump, place the output capacitor, COUT, as close to the OUT pin as possible. The ground terminal of COUT is connected to the nearest PGND or GND pin, or the exposed pad (EP).

　　To ensure the ground output impedance of the charge pump, the flying capacitors (C1 and C2) should be mounted as close to the IC as possible. If vias cannot be avoided, it is best to connect them in series with C1 and/or C2 instead of CIN or CINP, because flying capacitors have no effect on the stability of the device.

　　The ground end of any reference bypass capacitor (MAX1576 does not have this pin) or the ground end of the setting resistor (RM and RF of the MAX1576) should be connected to the GND pin (as opposed to PGND). This helps reduce the coupling noise of the IC's analog circuitry.

　　The exposed pad (EP) can optionally use large vias to facilitate inspection of the solder joint and to facilitate removal of the IC from the PCB with a soldering iron.

　　The logic inputs can be routed as needed to connect to the LEDs. Vias on these leads will not cause any problems because the current at these outputs is stable. However, be aware that if these leads are close to sensitive RF circuits, ripple on the leads may affect the RF circuits.