HT7466 查看數據表（PDF） - Holtek Semiconductor

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HT7466

3A Step-Down DC to DC Converter

Holtek Semiconductor 

HT7466

Functional Description

Setting the Output Voltage

The HT7466 is a step down switching DC/DC

converter device. The device switching output, SW, is

connected to a standard converter LC filter circuit for

buck conversion. The output current is sensed using

an external voltage divider network connected to the

load output voltage and ground, and the sense voltage

feedback to the device on the FB pin. By comparing

this sense voltage with an internally generated

1.222 voltage reference, suitable regulation can be

implemented to achieve the required output voltage.

By selecting suitable values of external voltage

divider resistors, the desired output voltage can be

adjusted to the required level. The output voltage can

be calculated using the following formula:

V OUT =

R1+ R2

R2

´ 1 .2 2 2

Where R2 is the resistor divider lower resistor and R1

is the upper resistor, and the R1, R2 must be kW level.

Compensation Components

Care must be taken when selecting external

components. The COMP pin is the output of the

error amplifier and must be properly connected to an

external RC network to ensure regulator loop stability.

The values of the compensation components given

in Table 1 yield a stable control loop for the output

voltage and capacitor given.

Table 1 Compensation Values for Typical Output

Voltage/Capacitor Combinations.

VOUT

L

R3 C4

R1

R2

C6

C5

2.5V 15mH 3.9kW 5.6nF 10.5kW 10kW 22mF Ceramic —

3.3V 15mH 4.7kW 4.7nF 17.0kW 10kW 22mF Ceramic —

5.0V 15mH 7.5kW 2.7nF 30.9kW 10kW 22mF Ceramic —

Bootstrap Circuit

In this device bootstrap capacitor is connected from

SW to BS pin, the Step-Down converter is use

N-Channel MOSFET, and MOS source pin is floating

to ground, this drive circuit of the application is

called high-side circuit, and the drive circuit used

in this kind of situation is called bootstrap circuit.

Large capacitor values can keep internal high-side

N-Channel MOSFET gate voltage greater than zero

when power MOS is turn on; usually a 10nF capacitor

is sufficient for most applications.

Current Limit Protection

The device has a cycle-by-cycle current limit to

protect the internal power MOSFET. If the inductor

current reaches the current limit threshold of 4.3A,

the MOSFET oscillator frequency will be transformed

from 380kHz to a short circuit frequency of 35kHz

and the output current will remain at its current limit

value.

Over Temperature Protection

A thermal shutdown is implemented to prevent

damage due to excessive heat and power dissipation.

Typically the thermal shutdown threshold is 150°C.

The thermal shutdown is triggered the device stops

switching and when the device thermal falls until the

temperature hysteresis windows the device will be

starts switching again.

Application Information

Inductor

The inductor is required to supply a constant current

to the output load while being driven by the switched

input voltage. The choice of inductor affects steady

state operation as well as transient behavior and

loop stability. There are three important electrical

parameters which need to be considered when

choosing an inductor:

• The inductor value

• DCR - copper wire resistance

• The saturation current

Inductor choice is especially important as it is

important to ensure the inductor does not saturate

under its peak current conditions, the general

rule of keeping the inductor current peak-to-peak

ripple approximately 30% of the nominal output

current. Using this value of ripple current is a good

compromise between excessive output voltage ripple

and excessive component size and cost. Also ensure

that the inductor has a low DCR to minimize power

losses.

Input/Output Capacitor

Input Capacitor

Because the Vin pin is connected to the internal power

MOSFET, when the power MOSFET is switching and

the input current is discontinuous, therefore an input

capacitor C1 (in PCB layout C1 and C8 is parallels

input capacitor) is required to supply the AC current

to the step-down converter while maintaining the

DC input voltage. A low ESR ceramic capacitor is

required to keep noise to a minimum.

Rev. 1.00

4

September 02, 2011

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