Data Sheet

LT8349

analog.com

Rev B

22 of 29

Output Capacitor Selection

The output capacitor has two essential functions. First, it filters the LT8349’s discontinuous top switch current to

produce the DC output. In this role, it determines the output ripple, thus low impedance at the switching frequency

is important. The second function is to store energy in order to satisfy transient loads and stabilize the IC’s control

loop. The X5R or X7R type ceramic capacitors have very low equivalent series resistance (ESR), which provides low

output ripple and good transient response. Transient performance can be improved with higher output capacitance

and the addition of a feedforward capacitor placed between VOUT and FB. When a feedforward capacitor is used or

output capacitance is adjusted, a careful evaluation of system stability should be made to ensure adequate design

margin. Increasing the output capacitance will also decrease the output voltage ripple. Lower value of output

capacitance can be used to save space and cost, but transient performance will suffer, and may result in loop instabil-

ity.

If there is significant inductance to the load due to long wires or cables, additional bulk capacitance may be

necessary. This can be provided with an electrolytic capacitor When choosing a capacitor, special attention should

be given to capacitor's data sheet to calculate the effective capacitance under the relevant operating conditions of

voltage bias and temperature

Frequency Compensation

The LT8349 has a VC pin which can be used to optimize the loop compensation. Designing the compensation network

is a bit complicated and the best values depend on the application and in particular the type of output capacitor. A

practical approach is to start with one of the circuits in the data sheet that is similar to your application and tune the

compensation network to optimize the performance. LTspice® simulations can help in this process. Stability should

then be checked across all operating conditions, including load current, input voltage, and temperature.

Figure 31 shows a simplified circuit for the LT8349 control loop. The error amplifier is a transconductance amplifier

(gm) generating output current IVC proportional to the voltage at FB pin. The two power sections, each consisting of a

power stage and an inductor, is modeled as a transconductance amplifier (gm1 or gm2) generating output current

(IOUT1 or IOUT2) proportional to the voltage at the VC pin. Note that the output capacitor COUT integrates IOUT1 and IOUT2,

and that the capacitor on the VC pin (CC) integrates the error amplifier output current IVC, resulting in two poles in the

loop. A zero is required and comes from a resistor RC in series with CC. This simple model works well as long as the

value of the inductor is not too high, and the loop crossover frequency is much lower than the switching frequency.

A small capacitor CC2 can be added to filter the switching noise that is coupled on the VC pin. A phase lead capacitor

C1 across R1 in the feedback divider can be used to improve the transient response and is required to cancel the

parasitic pole caused by the feedback node to ground capacitance.