STANDBY FUNCTION
The standby function, optimized for flyback topol-
ogy, automatically detects a light load condition
for the converter and decreases the oscillator fre-
quency on that occurrence. The normal oscillation
frequency is automatically resumed when the out-
put load builds up and exceeds a defined thresh-
old.
This function allows to minimize power losses re-
lated to switching frequency, which represent the
majority of losses in a lightly loaded flyback, with-
out giving up the advantages of a higher switching
frequency at heavy load.
This is accomplished by monitoring the output of
the Error Amplifier (V
COMP
) that depends linearly
on the peak primary current, except for an offset.
If the the peak primary current decreases (as a re-
sult 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 to a
lower value (f
SB
). When the peak primary current
increases and V
COMP
exceeds a second threshold
(V
T2
) the oscillator frequency is set to the normal
value (f
osc
). An appropriate hysteresis (V
T2
-V
T1
)
prevents undesired frequency change when
power is such that V
COMP
moves close to the
threshold. This operation is shown in fig. 21.
Both the normal and the standby frequency are
externally programmable. V
T1
and V
T2
are inter-
nally fixed but it is possible to adjust the thresh-
olds in terms of input power level.
APPLICATION INFORMATION
Detailed Pin Function Description
Pin 1.
SYNC (In/Out Synchronization). This func-
tion allows the IC’s oscillator either to synchronize
other controllers (master) or to be synchronized to
an external frequency (slave).
As a master, the pin delivers positive pulses dur-
ing the falling edge of the oscillator (see pin 2). In
slave operation the circuit is edge triggered. Refer
to fig. 23 to see how it works. When several IC
work in parallel no master-slave designation is
needed because the fastest one becomes auto-
matically the master.
During the ramp-up of the oscillator the pin is
pulled low by a 600
µ
A internal sink current gener-
ator. During the falling edge, that is when the
pulse is released, the 600
µ
A pull-down is discon-
nected. The pin becomes a generator whose
source capability is typically 7mA (with a voltage
still higher than 3.5V).
In fig. 22, some practical examples of synchroniz-
ing the L5991 are given.
Since the device automatically diminishes its op-
erating frequency under light load conditions, it is
reasonable to suppose that synchronization will
refer to normal operation and not to standby.
Pin 2.
RCT (Oscillator). Two resistors (R
A
and R
B
)
and one capacitor (C
T
), connected as shown in
fig. 23, allow to set separately the operating fre-
quency of the oscillator in normal operation (f
osc
)
and in standby mode (f
SB
).
C
T
is charged from Vref through R
A
and R
B
in nor-
mal operation (STANDBY = HIGH), through R
A
only in standby ( STANDBY = LOW). See pin 16
description to see how the STANDBY signal is gen-
erated.
When the voltage on C
T
reaches 3V, the capaci-
tor is quickly internally discharged. As the voltage
has dropped to 1V it starts being charged again.
1234
VCOMP
Pin
f
osc
f
SB
Stand-by
Normal operation
V
T
1
P
NO
P
SB
V
T
2
Figure 21. Standby dynamic operation.
L5991 L5991
R
A
VREF
SYNCSYNC
RCTRCT
L4981A
(MASTER)
L5991
(SLAVE)
R
A
VREFSYNC
RCT
R
OSC
C
OSC
C
T
L5991
(MASTER)
L4981A
(SLAVE)
SYNC
R
OSC
C
T
C
OSC
SYNC
(a) (b) (c)
R
A
D97IN728A
C
T
VREF
4
1
2
1
2
16
1817
4
2
1
RCT
12
4
16
17 18
ST-BY
16
R
B
ST-BY
16
R
B
R
B
16
ST-BY
Figure 22. Synchronizing the L5991.
L5991 - L5991A
8/23
