amp zero is equal to the complex pole frequency

LC of the inductor and output capacitor:

Input Capacitor Selection

the current peaks drawn from the input supply. The

source impedance to the input supply determines the

input capacitance. The input capacitor must meet the

switching currents. The RMS input ripple current is

given by:

For optimal circuit reliability, choose a capacitor that

has less than a 10°C temperature rise at the peak rip-

ple current.

Power MOSFET Selection

The MAX1966/MAX1967s’ step-down controller drives

two external logic-level N-channel MOSFETs. The key

selection parameters are:

rent limit and efficiency

3) Voltage rating and maximum input voltage

MOSFET Power Dissipation

Worst-case conduction losses occur at the duty factor

extremes. For the high-side MOSFET, the worst-case

power dissipation due to resistance occurs at minimum

input voltage:

The following switching loss calculation for the high-

side N-FET provides an approximation, but is no substi-

tute for evaluation:

(1A typical). For the low-side N-FET (N2), the worst-

case power dissipation occurs at maximum input volt-

age:

The low-side MOSFET on-resistance sets the

MAX1966/MAX1967 current limit. See the Setting the

Current Limit section for information on selecting low-

less, it is often possible to combine the high-side and

low-side MOSFETs into a single package (usually an 8-

pin SO) as indicated in Table 1. For higher output appli-

cations, or those where efficiency is more important,

separate FETs are usually preferred.

Very-Low-Voltage Applications

The MAX1966/MAX1967 are extremely versatile con-

trollers that can be used in a variety of applications

where high efficiency, high output power, and opti-

mized cost are important. One alternate connection,

shown in Figure 5, is useful when a low-voltage supply

is to be stepped down to an even lower voltage at high

current. If an additional bias supply is available, it can

supply gate drive separately from the input power rail.

This can either improve efficiency, or allow lower cost

5V logic-level MOSFETs to be used in place of 3V

MOSFETs.

P

V

V

IR

DN

OUT

VIN

LOAD

DS ON

()

(

)

2

1

2

=−









××

P

I

I

Vf

C

D N

SWITCHING

LOAD

GATE

VIN MAX

OSC

RSS

(/

)

(

)

1

2

=×

×

×

P

V

V

IR

D N RESISTIVE

OUT

VIN MIN

LOAD

DS ON

()

()

()

1

2

=×

×

II

VV

V

V

RMS

LOAD

OUT

VIN

OUT

VIN

=×

×−

()

C

LC

R

OUT

6

3

=

×

Low-Cost Voltage Mode PWM

Step-Down Controller

______________________________________________________________________________________

11

Figure 5. Low Input Voltage Step-Down with Extra Bias Supply

for Gate Drive

MAX1967

5V TO 28V FOR

GATE BIAS

VIN

COMP/EN

GND

BST

VCC

VL

DH

LX

DL

FB

3.3V

INPUT

R2

R1