NXP Semiconductors AN10881, Примечание к применению

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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 58 of 102
NXP Semiconductors
AN10881
TEA1713 resonant power supply control IC with PFC
8.6.2.3 SNSFB, SSHBC/EN and soft-start reset - operating frequency control
the SNSFB and SSHBC/EN can simultaneously control the operating frequency.
SSHBC/EN is dominant to provide protection and soft-start capability. Additionally, there is
an internal soft-start reset mechanism that overrules both SNSFB and SSHBC/EN control
inputs and immediately sets the frequency to maximum.
8.6.2.4 Soft-start reset
Some protections require a fast correction of the operating frequency to the maximum
value, but they do not have to stop switching. The overcurrent protection is an example
(see
Table 4 ).
When this protection is activated, the control input of the oscillator is disconnected
internally from the soft-start capacitor at pin SSHBC/EN and the switching frequency is
immediately set to maximum. In most cases, the change to the maximum switching
frequency restores safe switching operation. Once the voltage at pin SSHBC/EN has
reached 3.2 V, the control input of the oscillator reconnects to the pin and the normal
soft-start sweep follows.
Figure 35 shows the soft-start reset and the two-speed frequency
downward sweep.
The soft-start reset is also used to ensure a safe start-up at maximum frequency when the
HBC is enabled by SSHBC/EN or after a restart. The soft-start reset is not used when the
operation has been stopped for Burst mode.
Fig 35. Soft-start reset and two-speed soft-start
Protection
0
3.2 V
V
SSHBC/EN
8 V
0
f
min
f
HB
f
max
t
off
on
f
max
forced
fast
sweep
slow sweep regulation regulation
5.6 V
001aal045