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L6521
Low Voltage Smart Ballast Controller
March 2011 Doc ID 16998 Rev 3 1/19
L6520
L6521Highly integrated ballast controller for TL and CFL
Features Half bridge circuit able to drive both BJT and
MOSFET transistors Very accurate oscillator precision in wide
operating temperature range BJTs' storage time compensation Preheated start and instant start Hard switching protection Overcurrent / voltage protection Choke saturation control End-of-life protection Programmable without capacitors
Applications Electronic ballasts (TL, Industrial CFL) Integrated CFLs
Table 1. Device summary
Figure 1. Block diagram
Contents L6520, L65212/19 Doc ID 16998 Rev 3
Contents Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Functions description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95.1 Start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2 Preheating and instant start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.3 Ignition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.4 Run mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.5 Storage time compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.6 Current control circuit (CCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.6.1 Hard switching protection (HSP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.6.2 Overcurrent protection (OCPH) during ignition mode . . . . . . . . . . . . . . 11
5.6.3 Overcurrent protection (OCPL) during run mode . . . . . . . . . . . . . . . . . . 11
5.6.4 Choke saturation control (CSC) during ignition and run mode . . . . . . . 12
5.7 End of life (EOL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.8 Summary of protections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Typical electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Application examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
L6520, L6521 Description
Doc ID 16998 Rev 3 3/19
1 Description
The L6520/1 is the first highly integrated ballast controller in the market able to drive both
BJTs and MOSFETs, providing all the necessary protections to ensure the maximum
reliability of the application in compliance with major safety and power consumption
regulations.
By adopting BJTs switches in the application, the IC allows to replace more expensive
MOSFETs, strongly reducing the system cost without compromises.
The IC represents also the best and cost effective solution to replace self oscillating
solutions when the key requirement is the reliability of the ballast. The benefits are an
increased MTBF and a reduction of the costs due to the return from the field.
The higher level of flexibility and integration provided allows the possibility to quickly design
ballast with any kind of lamp topology/size/power, without limitations. Depending on the
power of the lamp, the IC can work without PFC, with passive PFC or with active PFC. In the
latter case the L6562A from STMicroelectronics is the suggested IC for the most cost
effective solution.
The IC is fully programmable using only resistors and offers over current protections, choke
saturation control and hard switching protection thanks to a sophisticated current control
circuit (CCC). In ignition, the CCC limits both the maximum lamp voltage in case of old or
broken lamp, and also the lamp current in case of inductor saturation.
When the IC is driving bipolar transistors, a variable dead time ensures the correct base
discharge time avoiding cross conduction phenomena. Moreover, the IC prevents the failure
due to the lamp's end of life (EOL).
Pin connection L6520, L6521
4/19 Doc ID 16998 Rev 3
2 Pin connection
Figure 2. Pin connection
Table 2. Pin description
L6520, L6521 Maximum ratings
Doc ID 16998 Rev 3 5/19
3 Maximum ratings
Table 3. Absolute maximum ratings VOUT refers to the voltage at either LVG pin or HVG pin
Table 4. Thermal data
Electrical characteristics L6520, L6521
6/19 Doc ID 16998 Rev 3
4 Electrical characteristics (a)
VCC = 16 V , TA = -25 °C to 85 °C, unless otherwise specified This is a preliminary version: all the parameters are subject to change
Table 5. Electrical characteristics
L6520, L6521 Electrical characteristics
Doc ID 16998 Rev 3 7/19
Table 5. Electrical characteristics (continued)
Electrical characteristics L6520, L6521
8/19 Doc ID 16998 Rev 3 During the operation at Vcc ≥ Vz the maximum supply current must be limited to 2mA. Guaranteed by characterization. tDEAD is the sum of a fixed time, generated by internal logic and the propagation delay of PWM_det comparator. Guaranteed by testing logic verification.
Table 5. Electrical characteristics (continued)
L6520, L6521 Functions description
Doc ID 16998 Rev 3 9/19
5 Functions description
5.1 Start-up
During the first start-up ramp of the supply voltage (VCC) both driver outputs, LSD and HSD,
are low impedance to ground (Isink 20 mA min). Once the VCC voltage reaches the turn-on
voltage VCC(ON) the IC starts its operation. During the first 100 µs the IC senses the status
of FPRE pin to detect the programmed preheating frequency and the selected ignition mode
(instant or preheated start). When all the IC internal functions are ready, the driver-outputs
are released.
If the preheated start is selected, the half-bridge oscillates at the programmed preheating
frequency, otherwise it starts from 85 kHz (typ.).
5.2 Preheating and instant start
The preheating time is 1.5 s (typ.) in the L6520 and 0.8 s (typ.) in the L6521. The FPRE pin
embeds a precise current reference: the voltage read by this pin sets the preheating
frequency or enables the instant start. If the FPRE pin is connected to ground, the instant
start is active and the IC runs immediately into ignition sequence from the starting frequency
of 85 kHz. If the pin FPRE is connected to a resistor equal or higher than 196 Ω , the
preheating frequency can be programmed from 55 kHz upwards till 100 kHz (1.5 kHz/step)
accordingly to Table 6. For the best precision the resistor tolerance should be less or equal
to 1%. After the preheating sequence, the IC runs into ignition mode.
Table 6. Preheating and instant start
Functions description L6520, L6521
10/19 Doc ID 16998 Rev 3
5.3 Ignition
During the ignition sequence the output frequency ramps down from the programmed
preheating frequency to the fixed run frequency with a fixed rate dfIGN/dt of - 2.75 kHz/ms. If
the instant start is selected, the frequency ramps down from 85 kHz to 46.6 kHz (typ.) with
the same rate.
The current control circuit limits the maximum lamp voltage (OCPH) in case of old or broken
lamp and it is able to control the lamp current in case of inductor saturation (CSC).
The ignition phase lasts for maximum 200 ms. If the Run frequency is not reached during
ignition phase, the IC is turned off (latched).
5.4 Run mode
The run frequency is internally set to 46.6 kHz.
The HSD and LSD pins drive respectively the high side and the low side switches. The
potential isolation to the high side switch is realized by a pulse transformer. The HSD and
LSD drivers are able to manage the inductive load represented by the primary side of the
pulse transformer.
Between the turn-off of one driver and turn-on of the other one there is a dead time
automatically optimized accordingly to the kind of the half bridge switches (MOS or BJT) to
ensure the maximum reliability. The CCC protects the circuit against over currents, choke
saturation and hard switching events.
5.5 Storage time compensation network
In all the operating states (preheating, ignition and run mode), the storage time
compensation ensures the application of the fixed dead time (tDEAD, 1.42 us typ.) once the
BJT's collector current is effectively reduced to zero. The tDEAD is the sum of a fixed time,
generated by internal logic and the propagation delay of PWM_det comparator.
The voltage level of the middle point of the half bridge is monitored through the PWM_det
pin: the high side switch is turned on after a fixed dead time from the instant when the
voltage on the PWM_det pin is above 2.65 V. The time between the falling edge of pin LSD
and the rising edge of HSD is recorded in order to set the same dead time between the
falling edge of pin HSD and the rising edge of pin LSD.
The minimum duration of the resulting ON time is internally limited to 1 µs. This condition
can last for a maximum time equal to 200 ms. After this time the IC is shut down (latched).
The PWM_det pin embeds a 5 V (typ.) clamping zener, allowing the connection between the
half bridge middle point and the pin itself by means of a limiting resistor.
When driving MOSFET no storage time is present, therefore the resulting dead time is equal
to (1.42 µs).
