Datasheet

L6738A Main oscillator
Doc ID 18134 Rev 2 19/32
7 Main oscillator
The controller embeds a programmable oscillator. The internal oscillator generates the
sawtooth waveform for the PWM charging with a constant current and resetting an internal
capacitor. The switching frequency, F
SW
, is internally fixed at 200 kHz.
The current delivered to the oscillator is typically 20 µA (corresponding to the free running
frequency F
SW
=200 kHz) and it may be varied using an external resistor (R
OSC
) typically
connected between the OSC pin and GND. As the OSC pin is fixed at 1.240 V, the
frequency is varied proportionally to the current sunk from the pin considering the internal
gain of 10 KHz/µA (see Figure 7).
Connecting R
OSC
to GND, the frequency is increased (current is sunk from the pin),
according to the following relationships:
Connecting R
OSC
to a positive voltage, the frequency is reduced (current is forced into the
pin), according to the following relationships:
where +V is the positive voltage to which the R
OSC
resistor is connected.
Figure 7. R
OSC
vs. switching frequency
F
SW
200kHz
1.240V
R
OSC
-------------------
10
kHz
µA
-----------
⋅+=
F
SW
200kHz
+V 1.240–
R
OSC
----------------------------
10
kHz
µA
-----------
⋅–=