L6730_09 Datasheet, PDF (14/52 Pages) STMicroelectronics – Adjustable step-down controller with synchronous rectification

Device description
5
Device description
L6730 - L6730B
5.1
Oscillator
The switching frequency is internally fixed to 400 kHz. The internal oscillator generates the
triangular waveform for the PWM charging and discharging an internal capacitor (F_SW = 400
kHz). This current can be varied using an external resistor (R_T) connected between OSC pin
and GND or V_CCDR in order to change the switching frequency. Since the OSC pin is
maintained at fixed voltage (typ. 1.2 V), the frequency is increased (or decreased)
proportionally to the current sunk (sourced) from (into) the pin. In particular by connecting
R_T versus GND the frequency is increased (current is sunk from the pin), according to the
following relationship:
Fsw = 400KHz + ^{9.88⋅106} (1)
R_OSC (KΩ)
Connecting R_T to V_CCDR reduces the frequency (current is sourced into the pin), according
to the following relationship:
Fsw = 400KHz − ^{3.01⋅107} (2)
R_OSC (KΩ)
Switching frequency variation vs. R_T is shown in Figure 4.
Figure 4. Switching frequency variation versus RT
1500
1400 _{Rosc connected to GND}
1300
1200
1100
1000
900
800
700
600
500
400
300
Rosc connected to Vccdr
200
100
0 100 200 300 400 500 600 700 800 900 1000
Rosc (KOHM)
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Doc ID 11938 Rev 3