LTM4663 datasheet(13/30 Pages) AD

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LT8722

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Rev. 0

• Fifth, ramp the output voltage control DAC (SPIS_DAC)

from code 0xFF000000 to code 0x00000000 in a con-

trolled manner so that the linear driver output (LDR)

ramps from GND to VIN/2. During this ramping period,

both the PWM driver output (SFB) and linear driver

output (LDR) move together to VIN/2. The ramp time

for this controlled movement to VIN/2 should be a mini-

mum of 5ms.

• Sixth, enable the PWM switching behavior by raising

the SWEN pin above the 1.25V threshold and writing

the SWEN_REQ bit to a 1. With both output terminals

at VIN/2, the inrush current through the output load

is greatly minimized. After the PWM driver switching

activity is enabled, keep the output voltage control DAC

(SPIS_DAC) code unchanged for a minimum of 160μs.

• Finally, the output voltage control DAC (SPIS_DAC)

code can be stepped in a controlled manner to the

desired code. The LDR and SFB outputs will begin to

diverge from one another until the desired differential

voltage is developed across the output load, the differ-

ential output voltage reaches the preset voltage limit,

or the output current reaches the preset current limit.

Figure 2 shows the flow chart of the enable sequence and

Figure 3 shows an example of the soft-start profile where

the instruction about soft-start guidance is followed.

BOOTSTRAP CAP

VOLTAGE REFRESH

EN PIN > 0.74V?

SUCCESSFULLY CLEAR

SPIS_STATUS

FAULT REGISTERS

VCC

REGULATOR ON

ENABLE_REQ = 1?

YES

LINEAR OUTPUT

ENABLED

YES

VIN AND VDDIO UP

8722 F02

VSFB AND VLDR

DIVERGE CONTROLLED

BY VDAC

ENABLE_REQ = 1?

AND SWEN > 1.125V

AND SWEN_REQ = 1?

LINEAR OUTPUT

STARTS CONTROLLED

BY VDAC

Figure 2. Flow Chart of Start-Up Sequence

1/2 VIN

8722 F03

VSFB

VLDR

VSFB FOLLOWS VLDR

VOUT BUILDS UP

Figure 3. Soft-Start Profile in Cooling Mode

POWERING THE DRIVERS

The LT8722 operates at an input voltage range of 3.1V to

15V that is applied to the VIN pin and an input range of

2.7V to 6V that is applied to the VDDIO pin.

The VIN pin is the power supply for the PWM driver and

the linear power driver. When configuring the power sup-

ply to the VIN pin keep in mind that, at high current loads,

the input voltage may drop substantially due to a voltage

drop in the wires between the front end power supply and

the VIN pin. Leave a proper voltage margin when design-

ing the front-end power supply to maintain good perfor-

mance. Minimize the trace length from the power supply

to the VIN pin to help mitigate the voltage drop.

SETTING THE SWITCHING FREQUENCY

The LT8722 uses a constant frequency PWM architec-

ture that can be programmed to switch from 500kHz to

3MHz through the SW_FRQ_SET register bits and further

be adjusted by ±15% through the SW_FRQ_ADJ regis-

ter bits. Table 1 and Table 2 show the frequency setup

summary.

Table 1. Switching Frequency Configuration

SW_FRQ_SET BITS

SWITCHING FREQUENCY

000

500kHz

001

1MHz

010

1.5MHz

011

2MHz

100

2.5MHz

101, 110, 11

3.0MHz

OPERATION

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