Data Sheet

LT8638-2

analog.com

Rev.0

17 of 35

To optimize efficiency at light loads, the LT8638-2 operates in Burst Mode operation in light load situations. Between

bursts, all circuitry associated with controlling the output switch is shut down, reducing the input supply current to

125µA (BIAS = 0). In a typical application, 90µA (VIN = 12V, BIAS = 5VOUT) is consumed from the input supply when

regulating with no load. The SYNC/ MODE pin is tied low to use Burst Mode operation and can be floated to use forced

continuous mode (FCM). If a clock is applied to the SYNC/MODE pin, the part synchronizes to an external clock

frequency and operates in FCM.

The LT8638-2 can operate in FCM for fast transient response and full frequency operation over a wide load range.

When in FCM, the oscillator operates continuously, and positive SW transitions are aligned to the clock. Negative

inductor current is allowed. The LT8638-2 can sink current from the output and return this charge to the input in this

mode, improving load step transient response.

To improve EMI, the LT8638-2 can operate in spread spectrum mode. This feature varies the clock with a triangular

frequency modulation of +24%. For example, if the LT8638-2’s frequency is programmed to switch at 2MHz, spread

spectrum mode modulates the oscillator between 2MHz and approximately 2.5MHz. The SYNC/MODE pin should be

tied high to INTVCC(or >3V) to enable spread spectrum modulation with forced continuous mode.

To improve efficiency across all loads, supply current to internal circuitry can be sourced from the BIAS pin when

biased at 3.3V or above. Else, the internal circuitry draws current from VIN. The BIAS pin should be connected to VOUT

if the LT8638-2 output is programmed at 3.3V to 25V.

The VC pin allows the loop compensation of the switching regulator to be optimized based on the programmed

switching frequency, allowing for a fast transient response. The VC and CLKOUT pins enable multiple LT8638-2

regulators to run with interleaving phase shift, reducing the amount of required input and output capacitors. The

PHMODE pin selects the phasing of CLKOUT for different multiphase applications.

Comparators monitoring the FB pin voltage pull the PG pin low if the output voltage varies more than ±7.75% (typical)

from the set point, or if a fault condition is present.

The oscillator reduces the LT8638-2 device’s operating frequency when the voltage at the FB pin is low. This

frequency foldback helps to control the inductor current when the output voltage is lower than the programmed

value, which occurs during start-up or overcurrent conditions. When a clock is applied to the SYNC/MODE pin, the

SYNC/MODE pin is floated, or held DC high, the frequency foldback is disabled, and the switching frequency slows

down only during overcurrent conditions.

APPLICATIONS INFORMATION

Low EMI PCB Layout

The LT8638-2 is specifically designed to minimize EMI emissions and to maximize efficiency when switching at high

frequencies. For optimal performance, the LT8638-2 should use multiple VIN bypass capacitors.

Two small 1µF capacitors should be placed as close as possible to the LT8638-2, one capacitor on each side of the

device (CIN1, CIN2). A third capacitor with a larger value, 4.7µF or higher, should be placed near CIN1 or CIN2.

See Figure 44 for a recommended PCB layout.

For more detail and PCB design files, refer to the demo board guide for the LT8638-2.

Note that large, switched currents flow in the LT8638-2 VIN and GND pins, and the input capacitors. The loops formed

by the input capacitors should be as small as possible by placing the capacitors adjacent to the VIN and GND pins.

Capacitors with small case size such as 0603 are optimal due to lowest parasitic inductance.