L5991 STMicroelectronics, L5991 Datasheet - Page 8

• Manufacturer Part Number: L5991
• Description: IC PRIMARY CTRLR W/STANDBY 16DIP
• Manufacturer: STMicroelectronics

Specifications of L5991

• Mfg Application Notes: 25W Off-Line Battery Charger with L5991A AppNote 300W Forward Converter with L5991A AppNote 45W AC-DC Adapter with Standby AppNote 60W Power Supply using the L5991 AppNote 80W Adapter using L6561/L5991A AppNote 90W SMPS for Monitors with Standby AppNote Trick Enhances L5991s Standby Function AppNote L5991-Based Converter, Temp Extra Power AppNote Auxiliary Power Supply Design AppNote Minimize Power Losses AppNote
• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 1MHz
• Duty Cycle: 93%
• Voltage - Supply: 11 V ~ 20 V
• Buck: No
• Boost: Yes
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: 0°C ~ 105°C
• Package / Case: 16-DIP (0.300", 7.62mm)
• Frequency-max: 1MHz
• Duty Cycle (max): 93 %
• Output Voltage: 5.075 V
• Output Current: 1500 mA
• Mounting Style: Through Hole
• Switching Frequency: 1000 KHz
• Operating Supply Voltage: 12 V to 20 V
• Maximum Operating Temperature: + 150 C
• Fall Time: 35 ns
• Minimum Operating Temperature: - 40 C
• Rise Time: 70 ns
• Synchronous Pin: Yes
• Topology: Boost, Flyback
• Package: DIP16 AND SO16
• For Use With: 497-8226 - BOARD EVAL L5991/STP12NK80Z497-6416 - BOARD EVAL L5991/STP10NK60Z497-5083 - EVAL BOARD 3PH PWR SUPPLY W/ESBT
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 497-7251-5; L5991

L5991 - L5991A
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
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
(V
lower value (f
increases and V
(V
value (f
prevents undesired frequency change when
power is such that V
threshold. This operation is shown in fig. 21.
Both the normal and the standby frequency are
externally programmable. V
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-
Figure 22. Synchronizing the L5991.
8/23
1
RCT
T1
T2
L5991
SYNC
), the oscillator frequency will be set to a
) the oscillator frequency is set to the normal
2
osc
16
4
R
ST-BY
B
). An appropriate hysteresis (V
VREF
(a)
R
COMP
C
A
SB
T
). When the peak primary current
COMP
falls below a fixed threshold
COMP
exceeds a second threshold
COMP
L5991
1
2
SYNC
RCT
) that depends linearly
T1
R
moves close to the
OSC
and V
(MASTER)
17
L4981A
18
16
T2
C
OSC
are inter-
SYNC
T2
(b)
-V
RCT
1
(SLAVE)
T1
L5991
ST-BY
2
)
16
4
Figure 21. Standby dynamic operation.
matically the master.
During the ramp-up of the oscillator the pin is
pulled low by a 600 A internal sink current gener-
ator. During the falling edge, that is when the
pulse is released, the 600 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
and one capacitor (C
fig. 23, allow to set separately the operating fre-
quency of the oscillator in normal operation (f
and in standby mode (f
C
mal operation (STANDBY = HIGH), through R
only in standby ( STANDBY = LOW). See pin 16
description to see how the STANDBY signal is gen-
erated.
When the voltage on C
tor is quickly internally discharged. As the voltage
has dropped to 1V it starts being charged again.
P
P
VREF
NO
SB
T
R
C
is charged from Vref through R
A
T
Pin
1
R
B
RCT
C
T
D97IN728A
R
B
2
R
2
(MASTER)
16
A
L5991
ST-BY
4
T
V
VREF
VCOMP
SB
1
), connected as shown in
T 1
T
Normal operation
).
reaches 3V, the capaci-
SYNC
3
SYNC
(c)
Stand-by
A
and R
R
16 17
OSC
(SLAVE)
L4981A
V
A
T 2
B
and R
4
in nor-
18
f
f
C
osc
SB
osc
OSC
B
A
)
)