LTM8021

7

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OPERATION

APPLICATIONS INFORMATION

For most applications, the design process is straight

forward, summarized as follows:

1. Refer to Table 1 for the row that has the desired input

range and output voltage.

3. Connect BIAS as indicated.

While these component combinations have been tested for

proper operation, it is incumbent upon the user to verify

proper operation over the intended system’s line, load and

environmental conditions.

If the desired output voltage is not listed in Table 1, set the

over the system’s intended line, load and environmental

conditions.

Minimum Duty Cycle

The LTM8021 has a fixed 1.1MHz switching frequency. For

any given output voltage, the duty cycle falls as the input

cycle can be very small. Because the LTM8021’s internal

controller IC has a minimum on-time, the regulator will

skip cycles in order to maintain output voltage regulation.

This will result in a larger output voltage ripple and pos-

sible disturbances during recovery from a transient load

step. The component values provided in Table 1 allow for

skip cycle operation, but hold the resultant output ripple

to around 50mV, or less. If even less ripple is desired,

then more output capacitance may be necessary. Adding

a feedforward capacitor has been empirically shown to

modestly extend the input voltage range to where the

LTM8021 does not skip cycles. Apply the feedforward

perturbations into the control loop, therefore, values

larger than 50pF are not recommended. A good value to

start with is 12pF.

The LTM8021 is a stand alone non-isolated step down

switching DC/DC power supply. It can deliver up to

500mA of DC output current with only bulk external input

and output capacitors. This module provides a precisely

regulated output voltage programmable via one external

to 36V. Given that the LTM8021 is a step down converter,

make sure that the input voltage is high enough to support

the desired output voltage and load current. Please refer

to the simplified Block Diagram.

The LTM8021 contains a current mode controller, power

switching element, power inductor, power Schottky diode

and a modest amount of input and output capacitance.

With its high performance current mode controller and

internal feedback loop compensation, the LTM8021 module

has sufficient stability margin and good transient perfor-

mance under a wide range of operating conditions with a

wide range of output capacitors, even all ceramic ones (X5R

or X7R). Current mode control provides cycle-by-cycle fast

current limit, and automatic current limiting protects the

module in the event of a short-circuit or overload fault.

The LTM8021 is based upon a 1.1MHz fixed frequency

PWM current mode controller, equipped with cycle skip

capability for low voltage outputs or light loads. A fre-

quency foldback scheme helps to protect internal com-

ponents from overstress under heavy and short-circuit

output loads.

The drive circuit for the internal power switching element

is powered through the BIAS pin. Power this pin with at

least 3V.