Datasheet LT1573 (Analog Devices) - 6

LT1573
W BLOCK DIAGRA
VOUT VIN 6 + + 7 I3 I2 SHDN 3 + NORMALLY NORMALLY + OFF OFF C3 S3 S2 C1 VIODTH VIN COMP FB GND 4 – – 6 8 1 VSHDNTH DRIVE 5 + + + I I LATCH I DISCHRG CHRG – R1 NORMALLY – OFF S4 2 LATCH S1 C2 + ERROR Q2 AMP R2 NORMALLY + ON + Q4 Q1 Q5 EXTERNAL VDRSAT C4 V Q3 LATCHTH + CAPACITOR 1.265V – CEXT FOR FIXED VOLTAGE VERSION THERMAL 4 GND 1573 BD SHUTDOWN
U U U FU CTIO AL DESCRIPTIO
The basic block diagram of the LT1573 is shown above. away before CEXT charges to the latch threshold, C2 will The regulating loop consists of a 1.265V reference, an switch back low, S1 will close and Q4 will discharge CEXT. error amplifier, a Darlington driver and an external PNP If the fault condition persists long enough for CEXT to pass transistor. The 1.265V reference feeds the noninvert- charge up to the latch threshold (VLATCHTH), comparator ing input of the error amplifier. The error amplifier drives C4 will switch high and S4 will close and latch the output the Darlington connected transistor pair Q1 and Q2. The off. The device will stay latched because latching current collector of Q1 comes out to the DRIVE pin and is used to ILATCH is greater than the pull-down current of Q4. Thermal drive the base of an external PNP power transistor as shutdown circuitry will close S4 and latch the device off shown in Figure 1. The error amplifier will adjust the drive with no timing delay. Comparator C1 is used to override current to the external PNP power transistor to maintain the latching function during start-up. If the difference the feedback pin voltage at 1.265V. The LT1573 provides between the output voltage and the input voltage is less overcurrent protection by means of a timed latch function. than the input-output differential threshold (VIODTH), Base current to the external PNP transistor is limited by comparator C1 output goes high which closes S2. Current placing a resistor between the base of the transistor and source I2 then drives base of Q4 which prevents CEXT from the DRIVE pin. When the DRIVE pin drops below VDRSAT charging. Comparator C3 is used for system shutdown (the DRIVE pin saturation voltage) the output of the and latch reset. If SHDN pin voltage is higher than shut- comparator C2 switches high; S1, which is normally down threshold VSDTH, the comparator C3 output goes closed, opens and the external capacitor connected to the high, shutting down the regulator and closing switch S3. LATCH pin is allowed to charge. Discharge current IDISCHRG Current I3 will drive Q4 to discharge CEXT, resetting the is equal to approximately 28µA and charging current ICHRG latch. is equal to approximately 7µA. If the fault condition goes sn1573 1573fas 6