Datasheet LT1573 (Analog Devices) - 9

LT1573
U U W U APPLICATIO S I FOR ATIO
Resistor RB helps to turn off the PNP (QOUT in Figure 2). RD = (4.5 – 0.95 – 0.39)V/(100 + 19)mA = 26.6Ω Smaller values for RB turn off the PNP faster but will The next lowest 5% value is 24Ω. increase input current. The recommended value for RB is 50Ω. For circuits that do not require high output current or
Overcurrent Latch-Off
fast transient response, the value of RB can be increased up to 200Ω. For the D45H11, the emitter-base voltage is In addition to limiting the base drive current, the resistor a function of base and collector current. Table 2 lists some RD is included in the circuit for the overcurrent protection useful operating points for the D45H11. These points were latch-off function. There is a minimum value for this empirically determined using a sampling of devices. resistance. It is calculated by Equation 1 with the drive current IDRIVE set to the minimum available drive current
Table 2. D45H11 VBE
(= 250mA) from the LT1573. At high currents, RD also
IB IC VBE AT 25
°
C
limits the power dissipation in the LT1573. In some
(mA) (A) (V)
conditions, resistor RD can be replaced with a short. This 1 0.2 0.65 is possible in circuits where an overload is unlikely and the 7 1 0.75 input voltage and drive requirements are low. If resistor RD 23 2 0.80 is not included in the circuit, the regulator is protected 45 3 0.85 against the overcurrent condition only by the thermal 66 4 0.90 shutdown function. After the resistor RD is determined, a 100 5 0.95 certain amount of base drive current is available to the external PNP transistor. An overcurrent or output short
Design Example
condition will demand a base drive current greater than the LT1573 can supply. The internal drive transistor will Given the following operating requirements: saturate. A time-out latch will be triggered by this 4.5V < V overcurrent condition to turn off the regulator system. The IN < 5.5V I time-out period is determined by an external capacitor OUT(MAX) = 5A V connected between the LATCH and GND pins. The time- OUT = 3.3V out period is equal to the time it takes for the capacitor to 1. The first step is to determine the required drive current charge from 0V to the latch threshold which is equal to for the D45H11. Dropout voltage must be less than 1.2V 2V at 5A output current. From Table 1, a drive current of BE. The latch charging current is set by an internal current source and is a function of input voltage and 100mA will give 0.7V dropout voltage at an output temperature as shown in the typical performance charac- current of 5A. This satisfies the operating require- teristics curve. At 25°C, the typical latch charging current ments. ranges from 7.2µA with 3V input to 8µA with 7V input. If 2. The next step is to determine the value of RD. Assume the overcurrent or output short condition persists longer RB is 50Ω. From Table 2, the maximum emitter-base than the time-out period, the regulator will be shut down. voltage for this design is 0.95V. The current through Otherwise, the regulator will function normally. In the RB is: latch-off mode, some extra current is drawn from the input to maintain the latch. The latching current is a function of IRB = VBE/RB = 0.95/50 = 19mA input voltage and temperature as shown in the typical VDRIVE is the DRIVE pin saturation voltage when the performance characteristic curve. At 25°C, the typical DRIVE pin current equals 119mA, which can be read latching current ranges from 0.3mA with 3V input to from the typical performance characteristics curve to 9.5mA with 7V input. The latch can be reset by recycling be 0.39V. Resistor RD now can be calculated from input power, by grounding the LATCH pin or by putting the Eq (1): device into shutdown. sn1573 1573fas 9