LP8557 datasheet(40/49 Pages) TI2

40 / 49 page

LP8557, LP85571

SNVSA15B – DECEMBER 2013 – REVISED DECEMBER 2015

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11.1.2 Schottky Diode Placement

In the LP8557’s 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 can cause a voltage spike (VSPIKE = LP_ × dI/dt) at SW and OUT. 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 to the SW pin as possible, and connecting the cathode of the diode

as close to COUT+ as possible, reduces the inductance (LP_) and minimize these voltage spikes.

11.1.3 Inductor Placement

The node where the inductor connects to the LP8557 SW bump has 2 challenges. First, a large switched voltage

(0 to (VOUT + VF_SCHOTTKY)) appears on this node every switching cycle. This switched voltage can be capacitively

coupled into nearby nodes. Second, there is a relatively large current (input current) on the traces connecting the

input supply to the inductor and connecting the inductor to the SW bump. Any resistance in this path can cause

voltage drops that can negatively affect efficiency and reduce the input operating voltage range.

To reduce the capacitive coupling of the signal on SW into nearby traces, the SW bump-to-inductor connection

must be minimized in area. This limits the PCB capacitance from SW to other traces. Additionally, high-

impedance nodes that are more susceptible to electric field coupling need to be routed away from SW and not

directly adjacent or beneath. This is especially true for traces such as ISET/SCL, FSET/SDA, and PWM. A GND

plane placed directly below SW dramatically reduces the capacitance from SW into nearby traces.

Lastly, limit the trace resistance of the VBATT-to-inductor connection and from the inductor-to-SW connection, by

use of short, wide traces.

11.1.4 Boost Input and VDD Capacitor Placement

The LP8557 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

MOSFET driver currents can add voltage spikes on the input due to the inductance in series with the VIN/VDD

and the input capacitor. Close placement of the input capacitor to the VDD pin and to the GND pin is critical

because any series inductance between VIN/VDD and CIN+ or CIN– and GND can create voltage spikes that

could appear on the VIN/VDD supply line and GND.

Close placement of the input capacitor at the input side of the inductor is also critical. The source impedance

(inductance and resistance) from the input supply, along with the input capacitor of the LP8557, forms a series

RLC circuit. If the output resistance from the source (RS, Figure 49) is low enough, the circuit is underdamped

and has a resonant frequency (typically the case). Depending on the size of LS, the resonant frequency could

occur below, close to, or above the LP8557's switching frequency. This can cause the supply current ripple to be:

1. Approximately equal to the inductor current ripple when the resonant frequency occurs well above the

LP8557 switching frequency.

2. Greater than the inductor current ripple when the resonant frequency occurs near the switching frequency.

3. Less than the inductor current ripple when the resonant frequency occurs well below the switching frequency.

Figure 49 shows the series RLC circuit formed from the output impedance of the supply and the input

capacitor.

The circuit is redrawn for the AC case where the VIN supply is replaced with a short to GND and the LP8557 +

Inductor is replaced with a current source (

ΔIL). Equation 1 is the criteria for an under-damped response.

Equation 2 is the resonant frequency. Equation 3 is the approximated supply current ripple as a function of LS,

RS, and CIN.

As an example, consider a 3.8-V supply with 0.1-

Ω of series resistance connected to CIN (10 µF) through 50 nH

of connecting traces. This results in an under-damped input-filter circuit with a resonant frequency of 225 kHz.

Because both the 1-MHz and 500-kHz switching frequency options lie above the resonant frequency of the input

filter, the supply current ripple is probably smaller than the inductor current ripple. In this case, using Equation 3,

the supply current ripple can be approximated as 0.2 times the inductor current ripple (using a 500-kHz switching

frequency) and 0.051 times the inductor current ripple using a 1-MHz switching frequency.

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