Datasheet LTC1701, LTC1701B (Analog Devices) - 5

LTC1701/LTC1701B
U OPERATIO
The LTC1701 uses a contant off-time, current mode archi- loop is enabled and the error amplifier drives the ITH/RUN tecture. The operating frequency is then determined by the pin. Soft-start can be implemented by ramping the voltage off-time and the difference between VIN and VOUT. on the ITH/RUN pin (see Applications Information sec- tion). The output voltage is set by an external divider returned to the VFB pin. An error amplfier compares the divided output
Low Current Operation
voltage with a reference voltage of 1.25V and adjusts the peak inductor current accordingly. To optimize efficiency when the load is relatively light, the LTC1701 automatically switches to Burst Mode operation
Main Control Loop
in which the internal PMOS switch operates intermittently based on load demand. The main control loop is inter- During normal operation, the internal PMOS switch is rupted when the output voltage reaches the desired regu- turned on when the VFB voltage is below the reference lated value. The hysteretic voltage comparator trips when voltage. The current into the inductor and the load in- I creases until the current limit is reached. The switch turns TH/RUN is below 1.5V, shutting off the switch and reduc- ing the power consumed. The output capacitor and the off and energy stored in the inductor flows through the inductor supply the power to the load until the output external Schottky diode into the load. After the constant voltage drops slightly and the I off-time interval, the switch turns on and the cycle repeats. TH/RUN pin exceeds 1.5V, turning on the switch and the main control loop which The peak inductor current is controlled by the voltage on starts another cycle. the ITH/RUN pin, which is the output of the error For reduced output ripple, the LTC1701B doesn't use amplifier.This amplifier compares the VFB pin to the 1.25V Burst Mode operation and operates continuously down to reference. When the load current increases, the FB voltage very low currents where the part starts skipping cycles. decreases slightly below the reference. This decrease causes the error amplifier to increase the ITH/RUN voltage
Dropout Operation
until the average inductor current matches the new load current. In dropout, the internal PMOS switch is turned on continu- ously (100% duty cycle) providing low dropout operation The main control loop is shut down by pulling the ITH/RUN with V pin to ground. When the pin is released an external resistor OUT at VIN. Since the LTC1701 does not incorporate an under voltage lockout, care should be taken to shut is used to charge the compensation capacitor. When the down the LTC1701 for V voltage at the I IN < 2.5V. TH/RUN pin reaches 0.8V, the main control
U U W U APPLICATIO S I FOR ATIO
The basic LTC1701 application circuit is shown in  V V   1  f IN OUT Figure␣ 1. External component selection is driven by the O = −  VIN + VD   TOFF  load requirement and begins with the selection of L1. Once L1 is chosen, the Schottky diode D1 can be selected Although the inductor does not influence the operating followed by CIN and COUT. frequency, the inductor value has a direct effect on ripple current. The inductor ripple current ∆IL decreases with
L Selection and Operating Frequency
higher inductance and increases with higher VIN or VOUT: The operating frequency is fixed by VIN, VOUT and the  V V  V V  constant off-time of about 500ns. The complete expres- ∆I IN OUT OUT D L = − + sion for operating frequency is given by:  fL  VIN + VD  where VD is the output Schottky diode forward drop. 5