Background technology
In LCD TV screen, the operating voltage of image converter (scaler), for considering of power saving and saving chip area, generally uses a lower operating voltage, is for example 3.3V.But, (back light) backlight light source of LCD TV screen, it may be light emitting diode (light-emitting diode, or cold-cathode fluorescence lamp (cold-cathode fluorescent lamp LED), CCFL), its operating voltage may be up to 40V.Therefore, just need to there is power supply changeover device (power converter), an operating power is transformed into another operating power.
A kind of power supply changeover device widely using is switch type power supplying device (switched-mode power supply, SMPS), because having outstanding conversion efficiency (power conversion efficiency).As shown in its name, SMPS has a power switch more, in order to control the energy storage of the Inductive component of flowing through or to release energy, to reach the object of converting electrical energy.
Just, this power switch need to bear quite high voltage mostly, therefore, and often also suitable height of the critical voltage of the control gate of power switch.With the signal that image converter was produced of operating power 3.3V power supply, its ceiling voltage may only have 3.3V, and is not enough to drive the control gate of general power switch.Therefore, need a level shifter (level shifter) or one drive circuit (driver), image converter is exported, belong to the driving signal of 3.3V system, the grid that are displaced to the electrical voltage system that a voltage is higher drive signal, use and drive a power switch.
Design when level shifter, need to be considered power supply order (power-on sequence), also need to consider can not wrong unlatching one power switch situation.If a power switch is opened by mistake and opened in power supply order process, may cause an Inductive component saturated, and cause danger.
Embodiment
Fig. 1 has shown according to the present invention and has implemented, and is applicable to the circuit of a LCD screen.Input voltage source VIN, is for example 12V, as the primary power (major voltage source) of whole circuit.Boost converter (booster) 10, on output terminal OUT, provides a higher voltage source, as driving voltage backlight.For example its voltage is 40V, in order to drive several LED serials (LED chains).Linear voltage decreasing device (low drop-out, LDO) 13, the voltage source V CC of generation one 3.3V, power supply is to image converter 20.Power supply order is considered, after being generally designed to voltage source V IN and being approximately ready for, for example the voltage of voltage source V IN is high to very having approached 12V, linear voltage decreasing device 13 just starts to make the voltage of voltage source V CC to rise, gradually draw and lift 3.3V from 0V, just drive image converter 20 to start working.
Image converter 20 can form with single-chip (monolithic chip), it has a pulse width modulation (pulse-width modulation, PWM) controller 18, in order to control the work period (duty ratio) of the power switch 15 in boost converter 10.In pulse width modulation controller 18, have a grid pre-driver (gate pre-driver) 16, the driving signal SDRV of its generation, is produced by voltage source V CC power supply station, belongs to the 3.3V system that voltage source V CC determines.In other words, drive the logic level of signal SDRV, will be between 0V between 3.3V.For instance, driving the low logic level of signal SDRV can be approximately 0V, and high logic level can be approximately 3.3V.
Level shifter 14 is coupled between grid pre-driver (gate pre-driver) 16 and power switch 15, in order to the control gate GATE of driving power switch 15, receive the driving signal SDRV that belongs to 3.3V system simultaneously, produce the grid driving signal SGATE that belongs to 12V system.For instance, it can be approximately 0V that grid drive the low logic level of signal SGATE, and high logic level can be approximately 11V.
Fig. 2 is the detailed circuit diagram of Fig. 1 level shifter 14.Level shifter 14 has input stage (input stage) 62 and output stage (output stage) 64, can form with separate type assembly (discrete device) assembling.From Fig. 2, can learn, except the driving signal SDRV as an input signal belongs to 3.3V system, the interior all signals of level shifter 14, no matter be reference voltage or import and export signal, be all to be produced by the voltage source V IN power supply of 12V, and be independent of the voltage source V CC of 3.3V in Fig. 1.Such benefit is: drive the voltage source V CC of signal SDRV not also to be ready for although power supply is given, namely its voltage is not yet increased to and approaches 3.3V, as long as voltage source V IN is stable, level shifter 14 is also just thereupon stable, prepares to drive signal SGATE according to driving signal SDRV to produce corresponding grid.
Input stage 62 is a grounded base amplifier (common-base amplifier).Input stage 62 comprises two junction transistor (bipolar-junction transistor, the BJT) BF of a PNP as input stage switch, and its emitter-base bandgap grading receives and drives signal SDRV.In the time that voltage source V IN stablizes, the voltage of contact N1 is set or provided to the strangulation voltage of Zener diode Z (clamping voltage), so also approximately determined the voltage of two junction transistor BF ground levels.The collector of two junction transistor BF, namely contact NC, sees through resistance R 1 and is couple to the voltage source V IN of 12V.In the time driving signal SDRV in a low logic level, such as when 0V, open (turn on) and operate in state of saturation as two junction transistor BF of switch, so contact NC can, at a very low voltage, be for example 0.2V.In the time driving signal SDRV in a high logic level, while being for example 3.3V, two junction transistor BF operate in closed condition, so contact NC is moved to a very high voltage by resistance R 1, may approach the voltage 12V of voltage source V IN.Therefore, the signal on contact NC has belonged to the 12V system that voltage source V IN determines.
As shown in Figure 2, output stage 64 is the plug-type grade follower (push-pull emitter follower) of penetrating.The two junction transistor BT of NPN are connected with the base stage of the two junction transistor BB of PNP, as an input end, are connected to contact NC.Two junction transistor BT are connected with the emitter-base bandgap grading of two junction transistor BB, and as an output terminal, output grid drive signal SGATE.The collector of two junction transistor BT is couple to voltage source V IN; The collector of two junction transistor BT is couple to ground wire.Grid drive the level of signal SGATE, will approximately follow (follow) contact NC level variation and change.So grid drive signal SGATE to belong to 12V system.Output stage 64 can amplify from contact NC to come and the electric current of input, produces amplified current, is exported by the emitter-base bandgap grading of two junction transistor BT or two junction transistor BB.Therefore output stage 64 can be considered as a current amplifier.
Can be known by inference by above analysis, in the time that voltage source V IN is ready for, the logic level of grid driving signal SGATE and driving signal SDRV can be consistent.For instance, when driving signal SDRV in logic " 0 " time, its magnitude of voltage is a low logic level (0V), grid drive the level of signal SGATE can, at another low logic level (0V), be in logic also " 0 ".When driving signal SDRV in a high logic level (3.3V), be in logic " 1 ", grid drive the level of signal SGATE can, another high logic level (may be 11V), be in logic also " and 1 ".
In one embodiment, one drive circuit only has an input stage and an output stage haply.On each signal path in every one-level, only has a driving component.For example, only have the two junction transistor BF of driving component on the signal path SP1 of the input stage 62 in Fig. 2, output stage 64 is having two signal path SPB and SPU, and two junction transistor BT are crossed in a meeting, and another can be through two junction transistor BB.In another embodiment, one drive circuit can have at least one buffer stage, is arranged between an input stage and an output stage.Follow considering of component count cost based on reducing signal delay, it is reasonable only having an input stage and an output stage.
Even if voltage source V IN is also unstable, as long as the voltage of voltage source V IN exceedes the strangulation voltage (as a default operating voltage) of Zener diode Z, Zener diode Z will be approximately by the ground level voltage stabilization of two junction transistor BF at a fixed voltage, guarantee that two junction transistor BF operate in state of saturation at the very start, and make contact NC maintain a lower voltage.So in power supply order process, contact NC will maintain a low voltage at the beginning always.This phenomenon can last till that voltage source V CC is ready for the electrical level rising of rear drive signal SDRV, and the voltage of contact NC just may be drawn high.
Level shifter 14 has following advantage:
1. can, in power supply order, not produce misoperation.The single voltage source V IN power supply of 14 needs of level shifter, produces any reference voltage or working point wherein.As previously mentioned, as long as the voltage of voltage source V IN exceedes the strangulation voltage of Zener diode Z, Zener diode Z will be by the ground level voltage stabilization of two junction transistor BF at a fixed voltage, guarantee that two junction transistor BF operate in state of saturation at the very start, and make contact NC maintain a lower voltage.If the 3.3V of voltage source V CC is not also ready for, drive the voltage of signal SDRV will approach very much 0V.Now, if the 12V of voltage source V IN is ready for, level shifter 14 is normal work just, and the logic level of the grid that it produces driving signal SGATE can be the same with driving signal SDRV, a namely low-voltage, is approximately grid and drives the low logic level of signal SGATE.Therefore, grid driving signal SGATE can be mistakenly in high logic level.
2. component count is few.As shown in Figure 2, only need to there be three BJT, a Zener diode and several resistance.The few awareness of defecation taste person low cost of component count.
3. operating speed is fast.Two junction transistor BF of input stage 62 are NPN, and its speed is generally higher than the two junction transistors of PNP.Output stage 64 is also the plug-type grade follower of penetrating of a high speed.So the operating speed of level shifter 14, more than can reaching 500kHz in theory.
Although the present invention is taking the circuit of LCD screen as example, the present invention goes for other application.As long as a signal that belongs to low voltage system being changed into a driving circuit that belongs to the driving signal of higher pressure system, can apply the present invention.
The foregoing is only preferred embodiment of the present invention, all equalizations of doing according to the present patent application the scope of the claims change and modify, and all should belong to covering scope of the present invention.