Datasheet LTC3534 (Analog Devices) - 9

LTC3534
operaTion
The LTC3534 provides high efficiency, low noise power The first circuit is an average current limit amplifier, sourc- for a wide variety of handheld electronic devices. Linear ing current out of FB to drop the output voltage should Technology’s proprietary topology allows input voltages the peak input current exceed 1.8A typical. This method above, below or equal to the output voltage by properly provides a closed loop means of clamping the input cur- phasing the output switches. The error amplifier output rent. During conditions where VOUT is near ground, such voltage on VC determines the output duty cycle of the as during a short circuit or start-up, this threshold is cut switches. Since VC is a filtered signal, it provides rejection to 800mA typical, providing a foldback feature. For this of frequencies from well below the switching frequency. current limit feature to be most effective, the Thevenin The low RDS(ON), low gate charge synchronous switches resistance from FB to ground should be greater than 100k. provide high frequency pulse width modulation control at Should the peak input current exceed 2.6A typical, the high efficiency. High efficiency is achieved at light loads second circuit, a high speed peak current limit compara- when Burst Mode operation is invoked and the LTC3534’s tor, shuts off PMOS switch A. The delay to output of this quiescent current drops to a mere 25µA. comparator is typically 50ns.
LOW NOISE FIXED FREQUENCY OPERATION Reverse Current Limit Oscillator
During fixed frequency operation, the LTC3534 operates in forced continuous conduction mode. The reverse cur- The frequency of operation is internally set to 1MHz. rent limit comparator monitors the inductor current from the output through PMOS switch D. Should this negative
Error Amplifier
inductor current exceed 500mA typical, the LTC3534 The error amplifier is a voltage mode amplifier. The loop shuts off switch D. compensation components are configured around the amplifier (from FB to V
Four-Switch Control
C) to obtain stability of the converter. For improved bandwidth, an additional R-C feedforward Figure 1 shows a simplified diagram of how the four internal network can be placed across the upper feedback divider switches are connected to the inductor, PVIN, VOUT, PGND1 resistor. The voltage on RUN/SS clamps the error amplifier and PGND2. Figure 2 shows the regions of operation for output, VC, to provide a soft-start function. the LTC3534 as a function of the internal control voltage, V
Supply Current Limits
CI. Dependent on the magnitude of VCI, the LTC3534 will operate in buck, buck-boost or boost mode. VCI is a level There are two different supply current limit circuits in the LTC3534, each having internally fixed thresholds. INTERNAL DUTY CONTROL CYCLE VOLTAGE, VCI 85% PV DMAX IN VOUT V4 (∼1.2V) BOOST 12 11 A ON, B OFF BOOST REGION PWM C AND D SWITCHES PMOS A PMOS D DMIN BOOST V3 (∼720mV) SW1 L1 SW2 FOUR SWITCH PWM BUCK-BOOST REGION DMAX V2 (∼640mV) 5 6 BUCK D ON, C OFF PWM A AND B BUCK REGION NMOS B NMOS C SWITCHES 0% V1 (∼100mV) 4 7 3534 F01 PGND1 PGND2 3534 F02
Figure 1. Simplified Diagram of Output Switches Figure 2. Switch Control vs Internal Control Voltage, VCI
3534fb For more information www.linear.com/LTC3534 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts