Power supplies that employ switching regulators often require tight control over the oscillator switching frequency, mainly in an effort to control high frequency noise that can interfere with sensitive circuitry. The LT1310 switching regulator can be synchronized to an external frequency, thus containing noise to well-defined frequency bands, which can be easily filtered.
The LT1310 combines a 1.5A Boost PWM DC/DC converter with an integrated phase-locked loop, which can be synchronized to any frequency between 10kHz and 4.5MHz. Figure 1 shows an application that converts 5V to 12V with an externally controlled switching frequency of 1.6MHz. To synchronize to an external input signal, the timing capacitor and PLL filter components must be chosen properly. This is a simple process and can be done using the graph in Figure 2.
In Figure 2, operating frequency is plotted versus timing capacitor (CT) with the upper and lower lines corresponding to the minimum and maximum lock frequency given a specific CT value. To choose the right timing capacitor, find the intersection of the desired operating frequency and the dashed line. Then move to the corresponding CT value.
Alternately, use the following equations as a starting point:
for fLOCK ≥ 2MHz:
for fLOCK ≤ 2MHz:
Because the lock range for the PLL is nearly 2:1, the nearest standard value NP0 capacitor can be used. For the application shown in Figure 1, a 1.6MHz switching frequency corresponds to an 100pF timing capacitor. Figure 3 shows the input frequency being stepped from 1.2MHz to 1.9MHz with the PLL regaining lock in approximately 50µs. Since the switching frequency affects inductor ripple current, the inductor must also be scaled. Table 1 shows recommended component values for various switching frequencies.