AN10907
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© NXP B.V. 2010. All rights reserved.
Application note
Rev. 1 — 28 December 2010
37 of 82
NXP Semiconductors
AN10907
TEA1613T resonant power supply control IC
8.4.4 RFMAX and High Frequency Protection (HFP)
Normally the converter does not operate continuously at the preset maximum frequency.
This maximum frequency is only used for a short time during soft-start or temporary
fault/overload conditions.
When the operating frequency remains at, or close to, maximum frequency for a longer
period, a fault condition is assumed and a protection activated.
For this, the HFP senses the voltage at pin RFMAX. This voltage indicates the actual
operating frequency. When the frequency is higher than approximately 75 % of the
frequency range (RFMAX = 1.83 V), the protection timer is started.
Be aware that during normal regulation the maximum frequency is limited to only 60 % of
the present range and V
RFMAX
is a maximum of 1.5 V.
8.5 HBC feedback (SNSFB)
A typical power supply application contains mains insulation in the HBC. On the
secondary (mains insulated) side, the output voltage is compared to a reference and
amplified. The TEA1613T is normally placed on the primary side. The output of the error
amplifier is transferred to the primary side via an optocoupler. The output of the
optocoupler on the primary side can be connected directly to SNSFB.
The SNSFB pin supplies the optocoupler from an internal voltage source of 8.4 V via an
internal series resistor of 1.5 k
. The internal series resistance also allows spike filtering
by an external capacitor at the pin.
To ensure sufficient bias current for proper working of the optocoupler, the feedback input
has a threshold current of 0.66 mA at which the frequency is minimum. The maximum
frequency controlled by SNSFB is reached at 2.2 mA. Notice that this is only
approximately 60 % of the total preset frequency range. The remaining upper part of the
present frequency range can only be reached by control of SSHBC/EN in case of
soft-start or protection.
Fig 23. Typical basic SNSFB application
001aal447
V+
8.4 V
3.2 V
SNSFB
V
OUT
CONTROL
TEA1613
1.5 k
Ω
