Datasheet LT1533 (Analog Devices) - 5

LT1533
U U U PIN FUNCTIONS COL A, COL B (Pins 2, 15):
These are the output collectors
GND (Pin 9):
Signal Ground. The internal error amplifier, of the power switches. Their emitters return to PGND negative feedback amplifier, oscillator, slew control cir- through a common sense resistor. COL A and cuitry and the bandgap reference are referred to this COL B are alternately turned on out of phase. Large ground. Keep the connection to the feedback divider and currents flow into these pins so it is desirable to keep VC compensation network free of large ground currents. external trace lengths short to minimize radiation. The
VC (Pin 10):
The compensation pin is used for frequency collectors can be tied together for simple boost applica- compensation and current limiting. It is the output of the tions. error amplifier and the input of the current comparator.
DUTY (Pin 3):
Tying the DUTY pin to ground will force the Loop frequency compensation can be performed with an outputs to switch with a 50% duty cycle. The DUTY pin RC network connected from the VC pin to ground. must float if not used.
SHDN (Pin 11):
The shutdown pin is used for disabling the
SYNC (Pin 4):
The SYNC pin can be used to synchronize switcher. Grounding this pin will disable all internal cir- the oscillator to an external clock (see Oscillator Sync in cuitry. Normally this output can be tied high (to VIN) or may Applications Information section for more details). The be left floating. SYNC pin may either be floated or tied to ground if not
RCSL (Pin 12):
A resistor to ground sets the current slew used. rate for the collectors A and B. The minimum resistor value
C
is 3.9k and the maximum value is 68k. Current slew will be
T (Pin 5):
The oscillator capacitor pin is used in conjunc- tion with R approximately: T to set the oscillator frequency. For RT = 16.9k, C I T(NF) = 129/fOSC(kHz) SLEW(A/µs) = 33/RCSL(kΩ)
R RVSL (Pin 13):
A resistor to ground sets the voltage slew
T (Pin 6):
The oscillator resistor pin is used to set the charge and discharge currents of the oscillator capacitor. rate for the collectors A and B. The minimum resistor value The nominal value is 16.9k. It is possible to adjust this is 3.9k and the maximum value is 68k. Voltage slew will be resistance approximately: ±25% to get a more accurate oscillator fre- quency. VSLEW(V/µs) = 220/RVSL(kΩ)
FB (Pin 7):
The feedback pin is used for positive voltage
VIN (Pin 14):
Input Supply Pin. Bypass this pin with a sensing and oscillator frequency shifting during start-up ≥ 4.7µF low ESR capacitor. When VIN is below 2.55V the and short-circuit conditions. It is the inverting input to the part will go into undervoltage lockout where it will stop error amplifier. The noninverting input of this amplifier output switching and pull the VC pin low. connects internally to a 1.25V reference. This pin should
PGND (Pin 16):
Power Switch Ground. This ground comes be left open if not used. from the emitters of the power switches. In normal opera-
NFB (Pin 8):
The negative voltage feedback pin is used for tion this pin should have approximately 25nH inductance sensing a negative output voltage. The pin is connected to to ground. This can be done by trace inductance (approxi- the inverting input of the negative feedback amplifier mately 1") or with wire or a specific inductive component. through a 100k source resistor. The negative feedback This inductance ensures stability in the current slew amplifier provides a gain of – 0.5 to the feedback amplifier. control loop during turn-off. Too much inductance (>50nH) The nominal regulation point would be – 2.5V on NFB. This may produce oscillation on the output voltage slew edges. pin should be left open if not used. 5