Datasheet LT1107 (Analog Devices) - 9

LT1107
O U U W U APPLICATI S I FOR ATIO
The usual step-up configuration for the LT1107 is shown VIN + R3 in Figure 1. The LT1107 first pulls SW1 low causing VIN – C2 100Ω VCESAT to appear across L1. A current then builds up in L1. ILIM VIN SW1 At the end of the switch ON time the current in L1 is1: FB LT1107 V L1 I I N = t SW2 VOUT PEAK ON ( ) 20 L GND R2 + D1 C1 L1 D1 1N5818 VIN VOUT R1 R3 R2 1107 F02 ILIM VIN SW1 +
Figure 2. Step-Down Mode Hookup
C1 LT1107 FB When the switch turns off, the SW2 pin falls rapidly and GND SW2 actually goes below ground. D1 turns on when SW2 R1 reaches 0.4V below ground. D1 MUST BE A SCHOTTKY DIODE. The voltage at SW2 must never be allowed to go 1107 F01 below –0.5V. A silicon diode such as the 1N4933 will allow
Figure 1. Step-Up Mode Hookup
SW2 to go to –0.8V, causing potentially destructive power Immediately after switch turn-off, the SW1 voltage pin dissipation inside the LT1107. Output voltage is deter- starts to rise because current cannot instantaneously stop mined by: flowing in L1. When the voltage reaches VOUT + VD, the  R2 inductor current flows through D1 into C1, increasing V 1 1 2 . V OUT = + 5 2 ( 3) V  R  ( ) 1 OUT. This action is repeated as needed by the LT1107 to keep VFB at the internal reference voltage of 1.25V. R1 and R3 programs switch current limit. This is especially im- R2 set the output voltage according to the formula: portant in applications where the input varies over a wide  range. Without R3, the switch stays on for a fixed time R2 V 1 1 2 . V each cycle. Under certain conditions the current in L1 can OUT = + 5 2 ( 1)  R  ( ) 1 build up to excessive levels, exceeding the switch rating and/or saturating the inductor. The 100Ω resistor pro-
Step-Down (Buck Mode) Operation
grams the switch to turn off when the current reaches A step-down DC/DC converter converts a higher voltage to approximately 700mA. When using the LT1107 in step- a lower voltage. The usual hookup for an LT1107 based down mode, output voltage should be limited to 6.2V or step-down converter is shown in Figure 2. less. Higher output voltages can be accommodated by inserting a 1N5818 diode in series with the SW2 pin When the switch turns on, SW2 pulls up to VIN – VSW. This (anode connected to SW2). puts a voltage across L1 equal to VIN – VSW – VOUT, causing a current to build up in L1. At the end of the switch
Inverting Configurations
ON time, the current in L1 is equal to: The LT1107 can be configured as a positive-to-negative V converter (Figure 3), or a negative-to-positive converter I N − V − V SW OUT I = t PEAK ON ( ) 22 (Figure 4). In Figure 3, the arrangement is very similar to L a step-down, except that the high side of the feedback is referred to ground. This level shifts the output negative. As
Note 1:
This simple expression neglects the effects of switch and coil in the step-down mode, D1 must be a Schottky diode, and resistance. This is taken into account in the “Inductor Selection” section. 1107fa 9