Electronic Components Datasheet Search |
|
LT1373CN8 Datasheet(PDF) 6 Page - Linear Technology |
|
LT1373CN8 Datasheet(HTML) 6 Page - Linear Technology |
6 / 12 page 6 LT1373 The LT1373 is a current mode switcher. This means that switch duty cycle is directly controlled by switch current rather than by output voltage. Referring to the Block Diagram, the switch is turned “On” at the start of each oscillator cycle. It is turned “Off” when switch current reaches a predetermined level. Control of output voltage is obtained by using the output of a voltage sensing error amplifier to set current trip level. This technique has several advantages. First, it has immediate response to input voltage variations, unlike voltage mode switchers which have notoriously poor line transient response. Second, it reduces the 90 °phaseshiftatmid-frequencies in the energy storage inductor. This greatly simplifies closed-loop frequency compensation under widely vary- ing input voltage or output load conditions. Finally, it allows simple pulse-by-pulse current limiting to provide maximum switch protection under output overload or short conditions. A low dropout internal regulator pro- vides a 2.3V supply for all internal circuitry. This low dropout design allows input voltage to vary from 2.7V to 25V with virtually no change in device performance. A 250kHz oscillator is the basic clock for all internal timing. It turns “On” the output switch via the logic and driver circuitry. Special adaptive anti-sat circuitry detects onset of saturation in the power switch and adjusts driver current instantaneously to limit switch saturation. This minimizes driver dissipation and provides very rapid turn-off of the switch. A 1.245V bandgap reference biases the positive input of the error amplifier. The negative input of the amplifier is brought out for positive output voltage sensing. The error amplifier has nonlinear transconductance to reduce out- put overshoot on start-up or overload recovery. When the feedback voltage exceeds the reference by 40mV, error amplifier transconductance increases ten times, which reduces output overshoot. The feedback input also invokes oscillator frequency shifting, which helps pro- tect components during overload conditions. When the feedback voltage drops below 0.6V, the oscillator fre- quency is reduced 5:1. Lower switching frequency allows full control of switch current limit by reducing minimum switch duty cycle. Unique error amplifier circuitry allows the LT1373 to directly regulate negative output voltages. The negative feedback amplifier’s 400k source resistor is brought out for negative output voltage sensing. The NFB pin regulates at – 2.45V while the amplifier output internally drives the FB pin to 1.245V. This architecture, which uses the same main error amplifier, prevents duplicating functions and maintains ease of use. (Consult Linear Technology Mar- keting for units that can regulate down to – 1.25V.) The error signal developed at the amplifier output is brought out externally. This pin (VC) has three different functions. It is used for frequency compensation, current limit adjustment and soft starting. During normal regula- tor operation this pin sits at a voltage between 1V (low output current) and 1.9V (high output current). The error amplifier is a current output (gm) type, so this voltage can be externally clamped for lowering current limit. Like- wise, a capacitor coupled external clamp will provide soft start. Switch duty cycle goes to zero if the VC pin is pulled below the control pin threshold, placing the LT1373 in an idle mode. Positive Output Voltage Setting The LT1373 develops a 1.245V reference (VREF) from the FB pin to ground. Output voltage is set by connecting the FB pin to an output resistor divider (Figure 1). The FB pin bias current represents a small error and can usually be ignored for values of R2 up to 25k. The suggested value for R2 is 24.9k. The NFB pin is normally left open for positive output applications. R1 VOUT = VREF 1 + R2 FB PIN VREF VOUT ()R1 R2 R1 = R2 – 1 () VOUT 1.245 LT1373 • F01 Figure 1. Positive Output Resistor Divider OPERATIO APPLICATIO S I FOR ATIO |
Similar Part No. - LT1373CN8 |
|
Similar Description - LT1373CN8 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |