Data Sheet

LT7200S

analog.com

Rev 0

20 of 35

INTV

CC Regulator Bypass Capacitor

An internal low dropout (LDO) regulator produces the 5V supply that powers the drivers and internal bias circuitry.

The INTVCC must be bypassed to ground with a minimum of 4.7μF ceramic capacitor. The decoupling capacitor

should have low impedance electrical connections to the INTVCC and GND pins to provide the transient currents

required by the LT7200S. Good bypassing is necessary to supply the high transient currents required by the power

MOSFET gate drivers. Applications with high input voltage and high switching frequency experience an increase in

die temperature due to the higher power dissipation across the LDO. In such cases, if there is another 5V supply rail

available, consider using that to drive the SVIN pin to lower the power dissipation across the internal LDO.

Output Voltage Programming

Each regulator’s output voltage is set by an external resistive divider according to the following equation:

VOUT = 0.5V x (1 +

R2

R1

)

The desired output voltage is set by the appropriate selection of resistors R1 and R2, which allow the VFB pin to sense

a fraction of the output voltage, as shown in Figure 33. Choosing large values for R1 and R2 results in improved

zero/light load efficiency but may lead to undesirable noise coupling or phase margin reduction due to stray

capacitances at the VFB node. Take care to route the VFB trace away from any noise source, such as SW trace. A

feedforward compensation capacitor, CFF, can also be placed between VOUT and FB to improve transient performance.

Figure 33. Setting the Output Voltage

If the output voltage is outside the VON sense range (0.5V ~ 4V), the output voltage stays in regulation, but the

switching frequency may deviate from the programmed frequency.

Soft-Start and Output Voltage TRACK

An internal 5μA pulls up the TRACK pin to INTVCC. Putting an external capacitor, CSS, from TRACK to ground enables

soft starting the output to prevent current surge on the input supply. The relationship between output rise time, TSS,

and soft-start capacitance, CSS, is given by

TSS = 1e5 x CSS

Upon start-up time, the LT7200S operates in discontinuous mode until track voltage is higher than 0.5V. The

regulator then operates in forced continuous mode until output is above the UV threshold (VFB > 0.4625V). Once the

output reaches this voltage, the operating mode of the regulator switches to the mode selected by the MODE/SYNC

pin as described above.

The LT7200S allows to program its output voltage ramp rate through the TRACK pin. From 0V to 0.5V, the TRACK

voltage overrides the internal 0.5V reference input to the error amplifier, thus regulating the feedback voltage to that

R1

R2

CFF

VOUT

KELVIN GND

LT7200S

FB

GND