E-L6598D013TR Datasheet, PDF (10/24 Pages) STMicroelectronics – High voltage resonant controller

Block diagram description
L6598
At the start-up (t = 0) the oscillator frequency is set by:
Equation 3
I_OSC(0)=
I_fmin + I_fstart
=
V_REF
⎛
⎝
------¹-------
R_fmin
+
-_R----_f--_s¹--_t--_a---_r--_{t⎠⎞}
At the end of soft start (t = T_SS) the second term of eq.1 decreases to zero and the switching
frequency is set only by Imin (i.e. R_fmin):
Equation 4
I_OSC(T_SS)=
I_fmin
=
-^V----^R----^E---^F-
R_fmin
Since the second term of eq.1 is equal to zero, we have:
Equation 5
I_fstart
–
g_----m----I_--s---s-
C_ss
T_S
S
=
0 → T_SS
=
C_-----s---s--I_{--f--s---t-a---r--t}
g_mI_ss
Note that there is not a fixed threshold of the voltage across C_SS in which the soft start
finishes (i.e. the end of the frequency shifting), and TSS depends on C_SS, Ifstart, g_m, and I_SS
(eq. 5). Making T_SS independent of Ifstart, the ISS current has been designed to be a fraction
of I_fstart, so:
Equation 6
I_SS
=
I_{--f--s---t-a---r--t}
K
→
T_SS
=
--^C----^s---^s---^I-^f--^s---^t-^a---^r--^t-
g_mI_fstartK
→
T_SS
=
--^C----^s---^s--
g_mK
→
T_SS
–
k_SSC_SS
In this way the soft start time depends only on the capacitor C_SS. The typical value of the k_SS
constant (Soft start timing constant) is 0.15 s/μF.
The current I_osc is fed to the oscillator as shown in fig. 7. It is twice mirrored (x4 and x8)
generating the triangular wave on the oscillator capacitor C_f. Referring to the internal
structure of the oscillator (fig.7), a good relationship to compute an approximate value of the
oscillator frequency in normal operation is:
Equation 7
f_min
=
-----¹---^.--⁴---¹-------
R_fminC_f
10/24
Doc ID 6554 Rev 7