AN937 Datasheet, PDF (4/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

Power supply, UVLO and voltage reference
2
Power supply, UVLO and voltage reference
AN937
The device is provided with an internal stabilized power supply (of about 12 V typ.) that
powers the analog and digital control blocks and the bootstrap section.
From this pre regulator, a 3.3V reference voltage ±2%, is internally available.
2.1
Oscillator and voltage feed forward
Just one pin is necessary to implement the oscillator function, with inherent voltage
feedforward.
Figure 4. Oscillator internal circuit
V_CC
R_OSC
TO PWM
COMPARATOR
Osc
Q₁
C_OSC
5R
-
+
R
Q₂
1V
CLOCK
D97IN655A
A resistor Rosc and a capacitor Cosc connected as shown in Figure 4, allow the setting of
the desired switching frequency in agreement with the below formula:
F_SW
=
------------------------------------------¹-------------------------------------------
(R_oasc
⋅
C_o
s
c⁾
In
⎛
⎝
65_{--⎠⎞}
+
100
⋅
C_osc
Where F_sw is in kHz, R_osc in kΩ and C_osc in nF.
The oscillator capacitor, C_osc, is discharged by an internal MOS transistor of 100 Ω of R_dson
(Q1) and during this period the internal threshold is set at 1 V by a second MOS, Q2. When
the oscillator voltage capacitor reaches the 1V threshold the output comparator turn-off the
MOS Q1 and turn-on the MOS Q2, restarting the C_osc charging.
The oscillator block, shown in Figure 4, generates a sawtooth wave signal that sets the
switching frequency of the system.
This signal, compared with the output of the error amplifier, generates the PWM signal that
will modulate the conduction time of the power output stage.
The way the oscillator has been integrated, does not require additional external components
to benefit of the voltage feed forward function.
4/29
Doc ID 5655 Rev 15