Features

Working under 2V to 30V Power supply voltage

Only 110 μA power supply current

Working in the voltage or lower voltage mode

] Only four external components

The low -battery detector comparator on the film

User -adjustable current limit

Internal 1A power switch

The output voltage version

8 -pin minidip or S8 package saving space

Palm on the palm

3V to 5V, 5V to 12V converter

9v 5V, 12V to 5V converter

LCD partial pressure generator

peripheral equipment and additional card

Backup battery

] Portable Instrument

Description

LT1108 is a multi -functional micro -power DC/DC converter. The device only requires four external components to provide fixed output of 5V or 12V. The power supply voltage range is 2V to 12V under the voltage range, and the 30V mode is in the lower voltage mode. The functions of LT1108 in the application of voltage, antihypertensive or reversal applications are equally good. The LT1108 is compatible with the tube feet compatible with the LT1173, but the duty cycle is 70%, thereby increasing the output in many applications. LT1108 can provide 150mA from 5V input from 2 AA batteries, and 300mA at 5V9 volt pressure mode. Static current is only 110 μA, making LT1108 an ideal choice for power conscious battery operating system. The switch current limits can be programmed as a resistor. Auxiliary gain blocks can be configured to low -battery detector, linear rear regulator, low -voltage locking circuit or error amplifier.

absolute value

Power voltage (VIN) 36 volt

SW1 pin voltage (vsw1) 50 volts

SW2 pin voltage (vsw2) - 0.5V to vin

Feedback pin voltage (LT1108) 5.5 volts

Detecting pin voltage (LT1108, -5, -12) 36 volt

Maximum power consumption 500 500 MGA

The maximum switching current 1.5 amp

The operating temperature range is 0 ° C to 70 ° C

Storage temperature range –65 ° C to 150 ° C

[

[ 123] Lead temperature (welding, 10 seconds) 300 degrees Celsius

Electric characteristics TA 25 ° C, vin 3V, unless otherwise explained.

Electrical C feature TA 25 ° C, vin 3V, unless there is another instructions

indicates the standard temperature range suitable for the entire operation.

Note 1: This specification guarantees that high and low checkpoints are within the range of 1.2V to 1.3V.

Note 2: Due to the following reasons, the output voltage waveform will present a sawtooth comparison delay. The output voltage on the fixed output publication will always be within the specified range.

Note 3: 100K resistance is connected between 5V power and A0 pin.

Ilim (pin 1): Connect this pin to Vin for normal use. Where? Require lower current restrictions, in Yilinhewen. 220 The resistor will limit the switch current to about 400 mAh.

vin (pin 2): Enter the power supply voltage.

SW1 (pin 3): power transistor set electrode. Used to upgrade mode to connect to the sensor/diode. Connect to VIN for the antihypertensive mode.

SW2 (pin 4): The power transistor emitting pole. Used to upgrade mode ground. For the antihypertensive mode, connect to the inductor/diode. You must not let this needle pipe fell under the ground.

GND (pin 5): ground.

AO (pin 6): Auxiliary gain block (GB) output. Open set electrode, can sink 100 μA.

set (pin 7): GB input. GB is a positive computing amplifier connected to the setting pin, and the negative input is connected to the 1.245V reference voltage.

FB/Sense (pin 8): On the LT1108 (adjustable), the pin is transferred to the comparator input end. In LT1108-5 and LT1108-12, the pin is connected to the internal application resistor to set the output voltage.

LT1108 is a door control oscillator switch. This type of architecture has a very low power current, because when the feedback pins voltage decreases, the switching cycle is lower than the reference voltage. It is best to understand the circuit run by referring to the LT1108 box diagram. Comparison A1 comparative feedback (FB) pin voltage 1.245V reference signal. When FB drops to 1.245V, A1 opens 19kHz oscillator. The amplifier increases the signal level to drive the NPN power switch. The switch cycle moves the output voltage and the FB pin voltage. When the FB voltage is enough to make the A1 stick, the oscillator is closed. A small amount of stagnation built in A1 ensures the external frequency compensation of the stability of the ring. When the output of the comparator is low, the oscillator and all high -current circuits are turned off, and the static current of the device is reduced to 110 micro -safety. The internal settings of the oscillator are connected to 36 μsTime and 17 μs shut down time, allowing many such as 2V to 5V converters. The continuous mode greatly increases the available output power.

gain block A2 can be used as a low battery detector. The negative input of this A2 is 1.245V reference voltage. The resistor

From VIN to GND, the midpoint is connected to the setting pin to provide a check -up voltage detector application when the battery power is low. A0 can absorb 100 μA (using 47K resistance, pull up to 5V). The maximum switching current connected between the ILIM pin and the VIN collection. When the switching current exceeds the set value, the switching cycle ends prematurely. If the current limit is not used, ILIM should be bundled directly to VIN. The transmission delay of the current limit circuit is about 2 μs. In the boost mode, the switching transmitter (SW2) is connected to the grounding and switch set electrode (SW1) driver -sensitive motor; in the antihypertensive mode, the collector is connected to the VIN transmitting polar driver sensor. LT1108-5 and LT1108-12 are functionally related to LT1108. -5 and -12 versions have the setting resistance of 5V or 12V output in the film. The fixed version of the pins 8 should be connected to the output. No external resistor is required.

Sensor selection

General Rules

The working principle of the dc/dc converter is to store energy as a magnetic flux in the inductive magnetic core core core In the middle, switch energy into the load. Because it is a flux, not the charge storage, by selecting the appropriate switching topology, the polarity of the output voltage and the input voltage can be higher, lower, or opposite. As an effective energy transmission component electrostatic sensor, three requirements must be met. First of all, the inductance must be low enough to make the sensor store sufficient space to minimize energy voltage and connection time in the worst case. The inductance must also be high enough, so the maximum current rated value is LT1108 in the worst case that does not exceed the maximum input voltage and connection time conditions of the sensor. In addition, the inductor core must be able to store the required flux; that is, not saturated. In the power level, the design of the LT1108 is usually encountered. Small surfaces are installed with saturated current iron oxygen magnetic core unit fixed values in the range of 300mA to 1A, and DCR is less than 0.4 (depending on the application) enough. Finally, the inductor must have a sufficiently low DC resistance so that too much power will not lose due to the calories in the winding. Another consideration is electromagnetic interference (EMI). Electromagnetic interference must be maintained, for example, a nearby circuit or sensor nearby with the minimum sensitivity. The rod core type is less expensive for EMI. The maximum input voltage, the output voltage and the output voltage inductor must be selected first.

Breathing converter

In the voltage or boost converter (Figure 1), the power generated by the inductors compensated input and output. The power required for the inductors is

Among them, VD is the diode voltage drop (1N5818 Schottky is 0.5V). The energy required per cycle of the sensor must be or greater than

to regulate the output of the converter. When the switch is turned off, the current increase in the inductor is based on

. R u0026#39; 8486;) and inductors DC resistance. Compared with VIN, the voltage drop on the switching is very small, and the simple non -destructive equation

can be used. When these equations assume that when T 0, the electrical sensor current is zero. The statement of the switch regulator is called ""discontinuous mode operation"". Set the ""T"" in the LT1108 specification table to the connection time (usually 36 μs), which will generate an iPeak or VIN with a specific ""L"". Once you know the iPeak, the end time of the energy gates in the inductor can be calculated as

EL must be greater than PL/FOSC to the required power to deliver. In order to achieve the best efficiency, the iPeak should be less than or equal to 1A. Higher switching current will lead to decreased efficiency through switches. Generally speaking, the switching current should be kept at least at least the loss of switches, diode and inductors. For example, it is assumed that 12V voltage will be generated from 2V to 3V at 30mA. Recall that the equation (01)

The energy required for the inductor is

Select 100 μH inductor value. As a result ]

Due to 18.3μJ u0026 GT; 16.6μJ, 100 μH sensor will work. This can be used for trial and error method for preferred sensors. Remember the maximum rated value of the switching current. If the peak current of the calculation exceeds this value, the external power transistor can be used. A resistor can connect with the Ilim tube foot to call the switch current limit. The selection of the resistor shall make the IPEAK calculated by the minimum vehicle identification number (VIN) is equal to the maximum value switch current (depending on the typical performance characteristic curve). Then, with the increase of VIN, the switching current remains unchanged, thereby improving efficiency.

Anti -pressure converter

The antihypertensive situation (Figure 2) Different from the voltage of the middle process, in these two processes, the electric sensor current flows through the charging cycle of the load sensor. It is currently limited to the ~ 650mA mode through the switch. By using the external switch (see Figure 3). ILIM pins are the key to successful input operations. After determining the output voltage, output current, and after input voltage range, the peak switch current can pass the formula:

ItsMid -DC DDM (0.60)

VSW The switch in VSW drop and switch in the antihypertensive mode drop

VD diode voltage drop (1N5818 is 0.5V)

out output Current

vout output voltage

vin minimum input voltage

vsw is actually a function of switching current

vin, L, time and vout's function. To simplify, 1.5V can be used as VSW as a very conservative value. Once you know the iPeak, the inductor value can be ton connect time (36μs) from

. Next, choose a restricted flow resistance RLIM to give the iPeak a lower -voltage mode curve from the RLIM. In addition, this resistor maintains the maximum switching current constant input voltage increase. For example, it is assumed that 5V voltage is generated from 12V to 24V when 300mA. Looking back (10),

Next, use equal (11) Calculator value

Use the next minimum standard value ( 330 μH).

Then select RLIM from the curve. For iPeak 500mA, rlim 220 .

Positive and negative converters

FIG. 4 shows the connection of positive and negative conversion. All output power must come from the inductors. In this case,

In this mode, the switch is arranged in a public collection appliance or antihypertensive mode. The switch can be modeled at 0.75V power supply with 0.65 the resistor is connected in series. When the switch is turned off, the current in the inductor is based on

due to 35.5 μJ u0026 GT; 28.9μJ, 220 μH sensor will work. Finally, you should select the RLIM current and the RLIM curve by viewing the switch. In this example, rlim 150 . The booster (booster mode) operates the boost type DC/DC converter provides the output voltage higher than the input voltage. The booster converter is not short -circuit protection, because the input end has DC channel output.

The commonly used voltage configuration of LT1108 is shown in

Figure 1. LT1108 first lowered SW1, causing Vin -Vcess to appear on L1. Then the current accumulates in L1. At the end of the connection time, the current in L1 is

After the switch is turned off, the SW1 voltage pin

Because the current cannot stop instantly, it begins to rise and flows in. L1. When the voltage reaches VOUT+VDAdd VOUT. LT1108 repeats this operation as needed to keep the internal reference voltage of the VFB 1.245V. The output voltage calculated based on the formula of R1 and R2 sets

Anti -voltage (antihypertensive mode) operation

Anti -voltage DC/DC converter will be higher The voltage is converted to a lower voltage. The common connection of LT1108 is shown in Figure 2. When the switch is turned on, SW2 will be pulled to vin -vsw. This is applied to the voltage equal to the VIN -VSW -VOUT on L1, causing the current formed in L1. At the end of the boot time, the current in L1 is equal to

The switch current limit of the program switch. This is particularly important in applications with large input changes. If there is no R3, each switch keeps open a fixed time cycle. Under certain conditions, the current in L1 can be established beyond the switch rated value and/or the sensor saturate. 100 The resistor to the ash of the resistor is about 700 mAh. When using LT1108 in the antihypertensive mode, the output voltage should be limited to 6.2V or below. The higher output voltage can be connected with the SW2 pin (anode connection to SW2) by inserting the 1N5818 diode

High -current antihypertensive operation

output current may be possible. Use discrete PNP to increase the transmission transistor as shown in Figure 3. R1 is used as a current restriction. When the voltage on R1 drops 0.5VBE, the switch is turned off. As shown in the figure, the switching current is limited to 2A. You can calculate the inductor value -based inductor selection step with the following VSW conservative expression converter segment

R2 provides the current path to turn off Q1. R3 provides the foundation to Q1. R4 and R5 set output voltage. PMOS field effect transistor can be used to replace Q1 when vehicle recognition number (VIN) is between 10 volts and 20 volts.

Reverse configuration

LT1108 can be configured as positive to negative converter (Figure 4), or negative-positive converter (Figure 5). In Figure 4, this arrangement and gradually decreased, but the feedback high -end finger ground. This level will output negatively. In the antihypertensive mode, the D1 must be the Schottky diode, and the u0026#63718; vout u0026#63718; should be less than 6.2V. More negative output, as mentioned in the previous section, can adjust the voltage. In Figure 5, the input is negative, and the output is positive. In this configuration, the input voltage can be higher than or below the output voltage. Horizontal conversion,

provided by the PNP transistor, provided correct polarity

Use Ilim pin

lThe T1108 switch is programmable for a set of switching current, which is a function that cannot be found on competitive devices. This allows the input to change a large range of more than the maximum switch rated or saturated sensor. Considering analysis shows that LT1108 must work at 2.0V input at the 800 mAh peak switch current. If VIN rises to 4V, the peak switch current will rise to 1.6A, exceeding the maximum switch current rated value. Select the appropriate resistor (see the ""maximum switching current VS RLIM"" feature), and even if the input voltage increases, the switch current will be limited to another situation of useful 800 mAh Ilim characteristics. This occurs in the following circumstances

When the input and output voltage meet this relationship, the inductor current is not zero within the switching time. When the switch is turned on again, the current slope starts with the non -zero current level of the sensor before the switch is turned on. As shown in Figure 6, the sensor current increases to a high horizontal closing the oscillator before the comparator. This high current can lead to excessive output ripples, and requires that the size of the output capacitor and inductors is too large. However, the use of the ILIM function switch current is turned off by programming level. As shown in the middle of the figure 7, keep the output ripple smallest

A very practical program

Figure 8 detailed the current limit circuit. Sensor Q1, the ratio of the basis and the power switch of the base and the power switch of the transmission pole is about 0.5%of the q2 set current flowing in the collector of Q1. This current is in the internal 80 resistor R1 and loses. The connected external resistance value is set up between ILIM and VIN. When enough, the switching current is used to form a VBE open on the R1+RLIM, the Q3 is formed and injects the current into the oscillator, and the switch is turned off. The delay of this circuit is about 2 μs. The current trigger point becomes inaccurate in the following circumstances that the time of inaccurate connection is less than 5 μs. Resistance programming ming connection time is 2 μs or less will cause response in the messy switch circuit to maintain the output adjustment.

Use gain block

The gain block (GB) on the LT1108 can be used as an error amplifier, a low -battery detector or a linear rear regulator. This gain block itself is a very simple PNP input computing amplifier, which has an open circuit set NPN output. The input of the negative input block is connected to the 1.245V reference voltage. Positive input is output on the set pin. The layout of the gain block as a low battery detector is straightforward. Figure 9 shows the connection. R1 and R2 only requires a sufficiently low value, so that the input of the biased current settings does not cause large errors. R2 is 33K enough. You can add R3 to introduce a small number of lags. This will cause the gain block ""capture"" to reach the trigger point. The range of 1 meter to 10 meters is the best range. However, the addition of R3 willChange the trigger point.