LT1108-12 Linear Technology, LT1108-12 Datasheet
LT1108-12
Related parts for LT1108-12
LT1108-12 Summary of contents
Page 1
... The LT1108 functions equally well in step-up, step- down, or inverting applications. The LT1108 is pin-for-pin compatible with the LT1173, but has a duty cycle of 70%, resulting in increased output current in many applications. The LT1108 can deliver 150mA at 5V from cell input and 5V at 300mA from 9V in step-down mode ...
Page 2
... C, JMAX 3V, unless otherwise noted CONDITIONS Switch OFF No Load LT1108-5 LT1108-12 Step-Up Mode Step-Down Mode LT1108 (Note 1) LT1108-5 (Note 2) LT1108-12 (Note 2) LT1108 LT1108-5 LT1108-12 Full Load, Step-Up Mode I Tied Step-Up Mode LIM IN LT1108 SET REF ...
Page 3
... LIM Quiescent Current 120 115 110 105 100 1000 –50 – TEMPERATURE (°C) UNITS 1000 LT1108 • TPC03 75 100 LT1108 • TPC06 3 ...
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... SET (Pin 7): GB input amp with positive input connected to SET pin and negative input connected to 1.245V reference. FB/SENSE (Pin 8): On the LT1108 (adjustable) this pin goes to the comparator input. On the LT1108-5 and LT1108-12, this pin goes to the internal application resistor that sets output voltage. Switch-ON Time 44 42 ...
Page 5
... V and the emitter drives the inductor. The LT1108-5 and LT1108-12 are functionally identical to the LT1108. The -5 and -12 versions have on-chip voltage setting resistors for fixed 5V or 12V outputs. Pin 8 on the fixed versions should be connected to the output ...
Page 6
... L can be used. These equations assume that inductor current is zero. This situation is called “discontinu- ous mode operation” in switching regulator parlance. Setting “t” to the switch-ON time from the LT1108 specifi- cation table (typically 36 s) will yield I and V . Once I IN ...
Page 7
... PEAK = switch-ON time (36 s). is selected to give I LIM Step-Down Mode curve. The addition of this LIM 2 300 500 – – . – 396 H 500 mA from the curve. For I = 500mA, LIM PEAK )( ) OUT D OUT LT1108 ( ) 11 PEAK (14) 7 ...
Page 8
... L1. When the voltage reaches (18) inductor current flows through D1 into C1, increasing V 220 H This action is repeated as needed by the LT1108 to keep V at the internal reference voltage of 1.245V. R1 and R2 set the output voltage according to the formula V OUT ...
Page 9
... D1 turns on when SW2 reaches 0.4V below ground. D1 MUST BE A SCHOTTKY DIODE . The voltage at SW2 must never be allowed to go below –0.5V. A silicon diode such as the 1N4933 will allow SW2 –0.8V, causing potentially destructive power dissipation inside the LT1108. Output voltage is deter- mined ...
Page 10
... Figure 5. Negative-to-Positive Converter USING THE I PIN LIM The LT1108 switch can be programmed to turn off at a set switch current, a feature not found on competing devices. This enables the input to vary over a wide range without exceeding the maximum switch rating or saturating the inductor. Consider the case where analysis shows the LT1108 must operate at an 800mA peak switch current with a 2 ...
Page 11
... OSCILLATOR Figure 8. LT1108 Current Limit Circuitry USING THE GAIN BLOCK The gain block (GB) on the LT1108 can be used as an error amplifier, low battery detector or linear post regulator. The gain block itself is a very simple PNP input op amp with an open collector NPN output. The negative input of the gain block is tied internally to the 1 ...
Page 12
... ZTX-949 OUT 200mA AT 6.5V 500mA AT 8V 100 100 + 1N5818 330 LIM 220 SW1 LT1108-5 SENSE LT1108 • TA04 SW2 * L1 = COILTRONICS CTX100-4 0.400 (10.160) MAX 0.250 ± 0.010 (6.350 ± 0.254 0393 0.189 – 0.197 (4.801 – 5.004) ...