lm5046sqx National Semiconductor Corporation, lm5046sqx Datasheet - Page 19

• Manufacturer Part Number: lm5046sqx
• Description: Phase-shifted Full-bridge Pwm Controller With Integrated Mosfet Drivers
• Manufacturer: National Semiconductor Corporation

Soft-Start of the Synchronous
Rectifiers
In addition to the basic soft-start already described, the
LM5046 contains a second soft-start function that gradually
turns on the synchronous rectifiers to their steady-state duty
cycle. This function keeps the synchronous rectifiers off dur-
ing the basic soft-start allowing a linear start-up of the output
voltage even into pre-biased loads. Then the SR output duty
cycle is gradually increased to prevent output voltage distur-
bances due to the difference in the voltage drop between the
body diode and the channel resistance of the synchronous
MOSFETs. Initially, when bias is supplied to the LM5046, the
Pre-Bias Startup
A common requirement for power converters is to have a
monotonic output voltage start-up into a pre-biased load i.e.
a pre-charged output capacitor. In a pre-biased load condi-
tion, if the synchronous rectifiers are engaged prematurely
they will sink current from the pre-charged output capacitors
resulting in an undesired output voltage dip. This condition is
undesirable and could potentially damage the power convert-
er. The LM5046 utilizes unique control circuitry to ensure
intelligent turn-on of the synchronous rectifiers such that the
output has a monotonic startup. Initially, the SSSR capacitor
is held at ground to disable the synchronous MOSFETs al-
lowing the body diode to conduct. The synchronous rectifier
soft-start is initiated once it is established the duty cycle is
controlled by the COMP instead of the soft-start capacitor i.e.
I
capacitor is then released and is charged by a 20µA current
source. Further, as shown in
SSSR pin is used to provide additional delay. This delay en-
COMP
< 800µA and the voltage at the SS pin>2V. The SSSR
Figure
7, a 1V offset on the
FIGURE 6. (a) Waveforms during Soft-Start
(b) Waveforms after Soft-Start
19
SSSR capacitor is discharged by an internal MOSFET. When
the SS capacitor reaches a 2V threshold and once it is es-
tablished that COMP is in control of the duty cycle i.e. I
< 800µA, the SSSR discharge is released and SSSR capac-
itor begins charging with a 20µA current source. During the
soft-start of synchronous rectifiers, SR1 and SR2 are turned
on simultaneously in pairs, as shown in
any transformer imbalance. The duty cycle of the SR outputs
gradually increase with increasing SSSR voltage until the du-
ty cycle gets locked into the steady state value as shown in
Figure
abled by grounding the SSSR pin.
sures the output voltage is in regulation avoiding any reverse
current when the synchronous MOSFETs are engaged.
Soft-Stop
As shown in
1.25V standby threshold, but above the 0.4V shutdown
threshold, the SSSR capacitor is soft-stopped with a 60µA
current source (3 times the charging current). Once the SSSR
pin reaches the 1.0V threshold, both the SS and SSSR pins
are immediately discharged to GND. Soft-stopping the power
converter gradually winds down the energy in the output ca-
pacitors and results in a monotonic decay of the output volt-
age. During the hiccup mode, the same sequence is executed
except that the SSSR is discharged with a 120µA current
source (6 times the charging current). In case of an OVP, VCC
UV, thermal limit or a VREF UV condition, the power converter
hard-stops, whereby all of the control outputs are driven to a
low state immediately.
6(b). The synchronous rectifier outputs can be dis-
Figure
8, if the UVLO pin voltage falls below the
Figure
6(a), to avoid
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COMP