Datasheet LT1185 (Analog Devices) - 5

LT1185
U U W U APPLICATIO S I FOR ATIO Block Diagram
A simplified block diagram of the LT1185 is shown in conditions, resulting in very high supply current when the Figure 1. A 2.37V bandgap reference is used to bias the input voltage is low. To avoid this situation, the LT1185 input of the error amplifier A1, and the reference amplifier uses an auxiliary emitter on Q3 to create a drive limiting A2. A1 feeds a triple NPN pass transistor stage which has feedback loop which automatically adjusts the drive to Q1 the two driver collectors tied to ground so that the main so that the base drive to Q3 is just enough to saturate Q3, pass transistor can completely saturate. This topology but no more. Under saturation conditions, the auxiliary normally has a problem with unlimited current in Q1 and emitter is acting like a collector to shunt away the output Q2 when the input voltage is less than the minimum current of A1. When the input voltage is high enough to required to create a regulated output. The standard “fix” keep Q3 out of saturation, the auxiliary emitter current for this problem is to insert a resistor in series with Q1 and drops to zero even when Q3 is conducting full load current. Q2 collectors, but this resistor must be low enough in value to supply full base current for Q3 under worst-case GND V R REF LIM (EXTERNAL) 2.37V REF FB – + – A1 A2 Q4 + Q1 VOUT D2 D4 D3 Q2 Q3 A5 A4 A3 – + + – + – I1 D1 R1 R2 300mV 2µA 350Ω 200mV 0.055Ω VIN LT1185 • BD
Figure 1. Block Diagram
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