AN10907
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© NXP B.V. 2010. All rights reserved.
Application note
Rev. 1 — 28 December 2010
41 of 82
NXP Semiconductors
AN10907
TEA1613T resonant power supply control IC
8.6.2.1
Soft-start voltage levels
At start-up, the SSHBC/EN voltage is low which corresponds to the maximum frequency.
During the soft-start procedure, the external capacitor is charged, the SSHBC/EN voltage
rises and the frequency decreases. The contribution of the soft-start function ends when
SSHBC/EN is above 8 V.
The SSHBC/EN voltage is clamped at 8.4 V and remains at that level during normal
operation.
When the voltage on SSHBC/EN is reduced during protection or regulation, the voltage is
clamped at 3.0 V. This is to provide a quick response so that the operating frequency can
be reduced again. Below 3.2 V the discharge current is reduced to 5
A.
8.6.2.2
SSHBC/EN charge and discharge
During initial start-up, the soft-start external capacitor on SSHBC/EN is only charged to
obtain a decreasing frequency sweep from maximum to operating frequency.
Besides the function to soft-start, SSHBC/EN is also used for regulation purposes such as
over-current regulation. Therefore the voltage on the capacitor on SSHBC/EN can vary by
charging and discharging it by internal current sources.
For example: in case of over-current regulation, a continuous alternation between
charging and discharging of the SSHBC/EN capacitor occurs. In this way the SSHBC/EN
voltage can be regulated, thereby overruling the signal on the feedback input SNSFB.
The (dis)charge current can have a high value
160
A or a low value
40
A. The
two-speed soft-start sweep of the TEA1613T allows a combination of a short start-up time
of the resonant converter and stable regulation loops such as overcurrent regulation.
In some cases there can be a situation when overcurrent regulation is activated during the
soft-start sequence. This results in a feedback controlled or corrected soft-start.
The fast (dis)charge speed is used for the upper frequency range where V
SSHBC/EN
is
below 5.6 V. In the upper frequency range the current and power in the converter do not
react strongly to fast frequency variations.
Fig 27. Operating frequencies related to SSHBC/EN voltage
f
HB
0
V
SSHBC
V
fmax(ss)
= 2.5 V
V
RFMAX
f
min
f
max
0
3.2 V = V
fmax(SSHBC)
8 V = V
fmin(SSHBC)
8.4 V = V
clamp(SSHBC)
3.0 V = V
pu(EN)
I
SNSFB
< 0.66 mA (not yet regulating)
0.66 mA < I
SNSFB
< 2.2 mA (regulating)
V
RFMAX
f
HB
001aal451
