Feature

2 internal switches

The working input voltage from 4 v to 36 v

3.3 v/(± 2%) reference voltage

] The output voltage can be adjusted from 1.235V to 35V

Low voltage difference operation: 100%duty cycle

500 kHz internal fixed frequency

Voltage feedback

] Air load current operation

Internal current limit

Zero current consumption suppression

synchronization

Feedback disconnection protection

Hot shutdown [heat 123]

Application

Consumer: set-top box, DVD, TV, VCR, vehicle radio, LCD display

Network: XDSL, modem, DC-DC module

Computer: printer, sound card/graphics card, disc storage, hard disk

Industry: charger, car battery, DC-DC converter

l5973ad [123 [123 ] It is a low -voltage single -chip power switch current limit of 2 A, so it can provide DC electric load current greater than 1.5 A depending on the application conditions. The output voltage can be set to 1.235V to 35 volts. Thanks to the high -current level, thanks to the exposed framework, it can be reduced, so that the Hong Kong Radio can be reduced to about 40 ° C/W. This device uses an internal P channel D-MOS transistor (typical value is 200 MΩ) as a switch to avoid the efficient rate of components using self-lifting capacitors. Internal oscillator fixed switching frequency is 500 kHz, minimizing external components. Its minimum input voltage is specially suitable for 5 V's bus, which has computer -related applications. The pulse current limit frequency modulation provides a valid constant current short circuit protection.

Electric characteristics

TJ 25 ° C, VCC 12 V, unless there are other regulations

Function description

The main internal module is shown in Figure 3, which reports the device box diagram.

They are:

The voltage regulator supplied to the internal circuit. From this regulator, a reference voltage is provided by a 3.3 伏 outer part.

Check the voltage monitoring circuit of input and internal voltage.

Full integrated sawtooth oscillator with a frequency of 500 kHz

Two embedded current restricted circuits for control current flow

Through the power switch. Pulse current limits the mandatory power switch

IfWhen the current reaches the internal threshold, it is turned off one by one, and the frequency

The gearrier reduces the switching frequency to greatly reduce the duty occupation ratio.

Cross -guidance error release large.

Pulse width modulator (PWM) comparator and related logic circuits drive the internal force.

High-voltage side drive of the internal P-MOS switch.

Forbidden blocks for spare operation.

Circuit to realize thermal protection function

Power and reference voltage

Internal regulator circuit (as shown in Figure 4) from start The circuit and internal voltage pre -adjustment device, the band gap base voltage, and the provision of currents with current. When the input voltage increases, the starter provides the start -up current to the entire device and the device is enabled (prohibited to pin the ground). The pre -regulator block provides a pre -regulating voltage VREG for the gap unit with very low power voltage noise sensitivity.

voltage monitor

Internal block continuous sensing VCC, VREF and VBG. If the voltage is higher than their threshold, the regulators start working. There is also a lag on VCC (UVLO).

oscillator and synchronization

FIG. 5 shows the frame diagram of the oscillator circuit. The clock generator provides internal fixing at 500 kg. In the case of strong cases, the frequency block plays an actual effect of reducing the switching frequency. The clock signal is then used in the input of an internal logic circuit slope generator and synchronous block. The slope generator circuit provides a sawtooth wave signal to achieve PWM control internal voltage feedback, and synchronous signals generate synchronous circuits. In fact, the device has a synchronous tube foot that can be at the same time. As the internal switching frequency of the external device synchronous external device. Synchronize as an external signal as a slave.

Especially connecting two devices together, one of the devices of one of the devices must be synchronized with the device, and the synchronous pipe foot must be transmitted from the low level to the high occupation ratio at 10%to the 10%to the 10%to the 10%to the 10%to the 10%to the 10%to the 10%to the 10%to the 10%to the 10%to the 10%to the 10%to the 10%. Stranger and frequency. The frequency of synchronous signals must be at least higher than the switch frequency of internal device (500 kHz).

current protection

L5973ad has two current limits protection, which is folded by pulse and frequency. The section of the current limiting circuit of the pulse is shown in Figure 6. The output power PDMOS transistor is divided into two parallel PDMOS. The smallest is a series resistor, RSENSE. The current is induced by the RSENSE. If the threshold is reached, the rearview mirror is unbalanced, and the PDMOS is turned off until the next decrease is down along the internal clock pulse. Due to the reduction of the connection time, the output voltage is reduced. Because the shortest connection time (what necessary to avoid false overcurrent signals) is not enough to obtain a low enough duty cycle at 500 kHz, the output current is overcurrent or short circuitThe situation may increase again. Because the frequency of this reason has also been reduced, maintaining the inductance current below its maximum threshold. The frequency shift (see Figure 5) depends on the feedback voltage. As a reduction of feedback voltage (due to the decrease in the duty -occupying ratio), the switching frequency is also reduced.

Error amplifier

Voltage error Player is the core of the ring regulation. This is a cross -guided non -inverter input connection to an internal voltage computing amplifier reference voltage (1.235V). When the reverse input (FB) is connected to the external frequency or directly to the output voltage. The output is connected to the external compensation network. No compensation error release large device has the following characteristics:

Error amplifier output compared with the oscillator sawtooth wave to achieve PWM control

PWM comparison device and power level

The output signal of this comparison oscillator sawtooth wave and error amplifier was generated to generate PWM signals. Power level is a very critical link because it must ensure the correct switch of PDMOS. Opening the power element, or more precisely, the rising time when the current is connected is a very critical parameter compromise. In the first method, the faster the rising time, the lower the loss. But there is a limit on the recovery time of the cycle diode. In fact, when the current of the power element is equal to the inductor current, the diode is turned off, and the drain power can be used freely. However, during its recovery time, the diode can be regarded as a high -value capacitor, which generates a very high peak current, which is responsible for many problems:

due to board parasitic. Turn on the current, leading to a decline in efficiency and system reliability. EMI problem is big. The life span of the continuity diode is shorter. The current decrease time during the cutting period is also critical. In fact, it generates a voltage peak (because of the parasitic element of the circuit board) to increase PDMOS. In order to reduce these problems, we have adopted a new driving circuit topological structure. The frame diagram is shown in Figure 7. The basic idea is to change the current level for opening and turning the power switch, according to the PDMOS status and door clamp status. This circuit allows fast shutdown and turning on the power switch, and manages the issues related to the recovery time of the two -polar pipe recovery time. To avoid the VGS of the internal switch higher than VGSMAX. Open/risk control blocks can avoid any cross -conducting and ground between power cords.

The inhibitory function

The prohibited function allows the device to place the device in the standby mode. The INH pin is more than 2.2 V. If the INH pin is kept floating, the internal pull -ups ensure that the voltage on the pin reaches the inhibitory threshold and the device is disabled. This pin is also compatible with VCC.

Hot shutdown

If the temperature is lower than the chip higher than the fixed internal thresholdValue (150 ° C). The sensor component chip is very close to the PDMOS area, so it ensures accurate and fast temperature testing. About 20 ° C's delay can avoid the opening and closing of the equipment continuously

additional functions and protection

Feedback disconnection

When the feedback is disconnected, it takes up The maximum value of the allowable ratio of the empty ratio makes the output voltage close to the input power supply. This situation will destroy loading. In order to avoid this dangerous situation, if the feedback pins still exist, the device floats.

Output overvoltage protection

Overvoltage protection, OVP, is achieved through an internal comparator. Its input is connected to feedback, when the OVP threshold is reached. This threshold is usually 30%higher than the feedback voltage. When the voltage division is required to adjust the output voltage (see the test application circuit), the OVP intervention will be set to:

. R2 is between feedback needles and ground.

empty load

Since the internal power supply is a PDMOS, it does not need a bootstrap capacitor. Therefore, the device can work normally when the output is not loaded. In this case, it can work a sudden mode and has a random sudden duplication rate.

Application circuit

FIG. 8 shows the application circuit of the demonstration board, where the input power supply voltage can be adjusted from 1.235V to VCC because the input capacitor and output voltage can be adjusted from 1.235V.

Please refer More application concepts.

L5973AD belongs to the L597X series.

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