AN1537 Datasheet, PDF (1/18 Pages) STMicroelectronics – A SIMPLE TRICK ENHANCES

AN1537
APPLICATION NOTE
A SIMPLE TRICK ENHANCES
L5991’s STANDBY FUNCTION
by C. Adragna and G. Gattavari
This application notes describes a simple technique that allows improving the Standby function of the
advanced PWM controller L5991. The price to pay for that is the addition of just two resistors and two
diodes, but the benefit brought in terms of no-load consumption in mains-operated converters is worth
this small fee. The effectiveness of the improved Standby function will be proved and assessed on a
couple of existing designs.
Introduction
L5991's Standby function is a valuable help in reducing light-load input consumption of offline converters and
making them compliant with energy saving standards such as EnergyStar, Energy2000 and others. This func-
tion, optimized for flyback topology, is the ability of automatically - and abruptly - reducing the oscillator frequen-
cy (i.e. converter's switching frequency) as the converter's load falls below a defined threshold and restoring the
normal oscillator frequency as the load increases and exceeds a second threshold.
The frequency shift allows minimizing power losses related to switching frequency, which represent most of
losses at light or no load, without giving up the advantages of a higher switching frequency at full load.
Being the L5991 a current-mode controller [1], the output voltage (V_COMP) of its error amplifier (pin 6, COMP),
except for an offset, is proportional to the peak primary current and then to the energy handled by the transform-
er cycle by cycle. It is then possible to deduce converter's load conditions by monitoring V_COMP.
Figure 1. L5991's Standby function operation: f_sw vs. V_COMP locus (left) and V_COMP vs. P_in locus (right).
fsw
Pin
fosc
Normal operation
PNO
Undershoot
during transition
Normal operation
fosc
fSB
fSB
1
Standby
2
VT1
3
VCOMP
VT2 4
PSB
1
Standby
Overshoot
during transition
2
VT1
3
VCOMP
VT2 4
If the peak primary current decreases as a result of a decrease of the power demanded by the load and V_COMP
falls below a fixed threshold (V_T1), the oscillator frequency will be set at a lower value (f_SB). If now the peak
primary current increases and V_COMP exceeds a second threshold (V_T2 > V_T1) the oscillator frequency will be
reset at the normal value (f_osc). Since the frequency shift causes V_COMP to shift too but in the opposite direction
for energy balance reasons, an appropriate hysteresis (V_T2-V_T1) is provided to prevent the oscillator frequency
from switching back and forth between f_SB and f_osc. This operation is shown in fig. 1.
The L5991 allows programming both the normal and the standby frequency. V_T1 and V_T2 are internally fixed but
May 2003
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