Datasheet LTM8053, LTM8053-1 (Analog Devices) - 19

link to page 23 link to page 23 link to page 19 link to page 23 link to page 19 link to page 19 LTM8053/LTM8053-1
APPLICATIONS INFORMATION
increased quiescent current. To enable pulse-skipping VIN mode, the SYNC pin is floated. The LTM8053-1 does not operate in discontinuous mode. VIN VOUT Instead, it operates in Burst Mode when SYNC is grounded, LTM8053 and in forced continuous mode when SYNC is floated or driven high and less than 4.2V. When an external SYNC GND NEGATIVE clock signal is applied, the LTM8053-1 operates in FCM. OUTPUT VOLTAGE 8053 F01a The LTM8053/LTM8053-1 features spread spectrum operation to further reduce EMI/EMC emissions. To en-
Figure 1a. The LTM8053/LTM8053-1 Can Be Used to Generate a Negative Voltage
able spread spectrum operation, apply between 2.9V and 4.2V to the SYNC pin. In this mode, triangular frequency VIN modulation is used to vary the switching frequency between the value programmed by RT to about 20% higher than VIN VOUT that value. The modulation frequency is about 3kHz. For LTM8053 example, when the LTM8053/LTM8053-1 is programmed to 2MHz, the frequency will vary from 2MHz to 2.4MHz at GND FAST LOAD TRANSIENT a 3kHz rate. When spread spectrum operation is selected, OUTPUT Burst Mode operation is disabled, and the part will run in TRANSIENT RESPONSE pulse-skipping mode. 8053 F01b The LTM8053 does not operate in forced continuous mode
Figure 1b. Any Output Voltage Transient Appears on
regardless of SYNC signal.
LTM8053/LTM8053-1 GND
FAST VIN
Negative Output
TRANSIENT OUTPUT EXPERIENCES VIN The LTM8053/LTM8053-1 is capable of generating a A POSITIVE TRANSIENT negative output voltage by connected its V VIN VOUT OUT to system GND and the LTM8053/LTM8053-1 GND to the negative LTM8053 CIN COUT voltage rail. An example of this is shown in the Typical GND Applications section. The most versatile way to generate a negative output is to use a dedicated regulator that AC DIVIDER OPTIONAL was designed to generate a negative voltage, but using a SCHOTTKY DIODE buck regulator like the LTM8053/LTM8053-1 to generate 8053 F01c a negative voltage is a simple and cost effective solution,
Figure 1c. A Schottky Diode Can Limit the Transient
as long as certain restrictions are kept in mind.
Caused by a Fast Rising VIN to Safe Levels
Figure 1a shows a typical negative output voltage applica- Note that this configuration requires that any load current tion. Note that LTM8053/LTM8053-1 VOUT is tied to system transient will directly impress the transient voltage onto GND and input power is applied from VIN to LTM8053/ the LTM8053/LTM8053-1 GND, as shown in Figure 1b, so LTM8053-1 VOUT. As a result, the LTM8053/LTM8053-1 fast load transients can disrupt the LTM8053/LTM8053-1’s is not behaving as a true buck regulator, and the maximum operation or even cause damage. Carefully evaluate output current is depends upon the input voltage. In the whether the negative buck configuration is suitable for example shown in the Typical Applications section, there the application. is an attending graph that shows how much current the The C LTM8053/LTM8053-1 deliver for given input voltages. IN and COUT capacitors in Figure 1c form an AC divider at the negative output voltage node. If VIN is hot-plugged Rev D For more information www.analog.com 19 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Package Photos Package Description Revision History Typical Application Related Parts