Datasheet LT1534 (Analog Devices) - 6

LT1534/LT1534-1
U U U PIN FUNCTIONS (LT1534/LT1534-1) COL (Pins 2, 15/Pin 2):
These two pins should be con- referred to this ground. Keep the connection to the feed- nected together externally to create the collector of the back divider and VC compensation network free of large power switch. The emitter returns to PGND through a ground currents. sense resistor. Large currents flow into these pins so it is
VC (Pin 10/Pin 11):
The compensation pin is used for desirable to keep external trace lengths short to minimize frequency compensation and current limiting. It is the radiation. output of the error amplifier and the input of the current
SYNC (Pin 4):
The SYNC pin can be used to synchronize comparator. Loop frequency compensation can be per- the oscillator to an external clock (see Oscillator Sync in formed with an RC network connected from the VC pin to Applications Information section for more details). The ground. SYNC pin may either be floated or tied to ground if not
SHDN (Pin 11/Pin 12):
The shutdown pin is used for used. disabling the switcher. Grounding this pin will disable all
C
internal circuitry. Normally this output can be tied high (to
T (Pin 5):
The oscillator capacitor pin is used in conjunction with R V T to set the oscillator frequency. IN) or may be left floating. For RT = 16.9k,
RCSL (Pin 12/Pin 13):
A resistor to ground sets the current CT(NF) = 129/fOSC(kHz) slew rate for the power switch. The minimum resistor value is 3.9k and the maximum value is 68k. Current slew
RT (Pin 6):
The oscillator resistor pin is used to set the will be approximately: charge and discharge currents of the oscillator capacitor. The nominal value is 16.9k. It is possible to adjust this ISLEW(A/µs) = 33/RCSL(kΩ) resistance ±25% to get a more accurate oscillator fre-
RVSL (Pin 13/Pin 14):
A resistor to ground sets the voltage quency. slew rate for the power switch collector. The minimum
FB (Pin 7):
The feedback pin is used for positive voltage resistor value is 3.9k and the maximum value is 68k. sensing and oscillator frequency shifting during start-up Voltage slew will be approximately: and short-circuit conditions. It is the inverting input to the VSLEW(V/µs) = 220/RVSL(kΩ) error amplifier. The noninverting input of this amplifier
V
connects internally to a 1.25V reference. This pin should
IN (Pin 14/Pin 15):
Input Supply Pin. Bypass this pin with a ≥ 4.7µF low ESR capacitor. When V be left open if not used. IN is below 2.55V the part will go into undervoltage lockout where it will stop
NFB (Pin 8/Pin 10):
The negative voltage feedback pin is output switching and pull the VC pin low. used for sensing a negative output voltage. The pin is
PGND (Pin 16/Pin 3):
Power Switch Ground. This ground connected to the inverting input of the negative feedback comes from the emitters of the power switches. In normal amplifier through a 100k source resistor. The negative operation this pin should have approximately 25nH induc- feedback amplifier provides a gain of – 0.5 to the feedback tance to ground. This can be done by trace inductance amplifier; therefore, the nominal regulation point is – 2.5V (approximately 1") or with wire or a specific inductive on NFB. This pin should be left open if not used. component (e.g., small ferrite bead). This inductance
GND (Pin 9/Pins 1, 8, 9, 16):
Signal Ground. The internal ensures stability in the current slew control loop during error amplifier, negative feedback amplifier, oscillator, turn-off. Too much inductance (>50nH) may produce slew control circuitry and the bandgap reference are oscillation on the output voltage slew edges. 6