Table 1. Application Example

PARAMETER	Buck1	Buck2	Buck3
Input voltage, VIN	6 V to 18 V 14 V, typical	6 V to 18 V 14 V, typical	4 V to 10 V 5 V, typical
Output ripple voltage	±0.2 V	±0.2 V	±0.1 V
Output voltage, VOUT	5 V ±2%	3.3 V ±2%	1.8 V ±2%
Maximum output current, IOUT	4.5 A	4.5 A	2.2 A
Minimum output current, IOUT	0.1 A	0.1 A	0.1 A
Load-step output tolerance, ∆VOUT + ∆VOUT(Ripple)	±0.3 V	±0.3 V	±0.15 V
Current-output load step, ∆IOUT	0.1 A to 4.5 A	0.1 A to 4.5 A	0.1 A to 2.2 A
Converter switching frequency, fSW	400 kHz	400 kHz	400 kHz
Junction temperature, TJ	125°C	125°C	125°C

8.2.2.1 High- and Low-Side Power NMOS Selection for the Buck Converters

An internal supply, which is 5.8 V typical under normal operating conditions, provides the gate-drive supply for these MOSFETs. The output is a totem pole, allowing full voltage drive of VREG to the gate with a peak output current of 0.6 A. The high-side MOSFET reference is the phase terminal (PHx), and the low-side MOSFET referenced is the power ground (PGNDx) terminal. For a particular application, select these MOSFETs with consideration for the following parameters r_DS(on), gate charge Qg, drain-to-source breakdown voltage BVDSS, maximum dc current I_DC(maximum), and thermal resistance for the package.

Power dissipation on the high-side FET (P_{D_HS}):

First term is conduction losses.

Second term is switching losses.

Power dissipation on the low-side FET (P_{D_LS}):

The first term in the foregoing equation refers to conduction losses, and the second term covers the switching losses in the FET body diode during the dead-time.

NOTE

r_DS(on) has a positive temperature coefficient TC, which is typically 0.4%/°C.

Gate losses for high-side and low-side FETs:

8.2.2.2 Buck1 Component Selection

Duty Cycle

Selection of Current Sensing Resistor

Use 10 mΩ to allow for ripple-current.

Inductor Selection L

Use 8.2 µH.

Inductor Ripple Current

Output Capacitor C_OUT

Use 100 µF.

Input Capacitor C_IN

Use 10 µF, shared between Buck1 and Buck2.

High-Side MOSFET (Buck1TOPFET)

Low-Side MOSFET (Buck1LOWFET)

8.2.2.3 Buck2 Component Selection

Duty Cycle

Selection of Current-Sensing Resistor

Use 10 mΩ to allow for ripple current.

Inductor Selection L

Use 8.2 uH.

Inductor Ripple Current

Output Capacitor C_OUT

Use 100 µF.

Input Capacitor C_IN

Use 10 µF, shared between Buck1 and Buck2. For better line-transient immunity, use a larger value.

High-Side MOSFET (Buck2TOPFET)

Low-Side MOSFET (Buck2LOWFET)

8.2.2.4 Buck3 Component Selection

Duty Cycle

Inductor Selection L_Buck3

Use 8.2 µH.

Inductor Ripple Current

Output Capacitor C_OUT

Use 100 µF.

Input Capacitor C_IN

Use 10 µF.

Internal High-Side MOSFET (Buck3TOPFET)

Internal Low-Side MOSFET (Buck3LOWFET)