NXP Semiconductors AN10881, Application Note, Page: 56 / 102

AN10881 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved.
Application note Rev. 2 — 26 September 2011 56 of 102
NXP Semiconductors
AN10881
TEA1713 resonant power supply control IC with PFC
During soft-start, in parallel to the soft-start frequency sweep, the SNSCURHBC function
monitors the primary current and can activate regulation in a (temporary) overpower
situation.
The soft-start uses the voltage on pin SSHBC/EN. an external capacitor on SSHBC/EN
sets the timing (duration) of the soft-start event.
As the SSHBC/EN is also used as enable input, the soft-start functionality is above the
enable related voltage levels (see
Figure 33 ).
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
Initially, at 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
overcurrent 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 overcurrent regulation, a continuous alternation between charging
and discharging of the SSHBC/EN capacitor occurs. The SSHBC/EN voltage can be
regulated in this way overruling the signal on the feedback input SNSFB.
Fig 33. 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
001aal043