HT7939

Rev 1.30

4

November 9, 2010

Function Description

VIN Under-Voltage Lockout

- UVLO

The device contains an Input Under Voltage Lockout

(UVLO) circuit. The purpose of the UVLO circuit is to en-

sure that the input voltage is high enough for reliable op-

eration. When the input voltage falls below the under

voltage threshold, the internal FET switch is turned off. If

the input voltage rises by the under voltage lockout hys-

teresis, the device will restart. The UVLO threshold is

set below the minimum input voltage of 2.6V to avoid

any transient VIN drops under the UVLO threshold and

causing the converter to turn off.

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, the MOSFET will be

turned off. It is import to note that this current limit will not

protect the output from excessive current during an out-

put short circuit. If an output short circuit has occurred,

excessive current can damage both the inductor and di-

ode.

Over-Voltage Protection

- OVP

The device provides an over-voltage protection func-

tion. If the FB pin is shorted to ground or an LED is dis-

connected from the circuit, the FB pin voltage will fall to

zero and the internal power MOSFET will switch with its

full duty cycle. This may cause the output voltage to ex-

ceed its maximum voltage rating, possibly damaging the

IC and external components. Internal over-voltage pro-

tection circuitry turns off the power MOSFET and shuts

down the IC as soon as the output voltage exceeds the

the level of the input supply voltage. The device remains

in shutdown mode until the power is recycled.

Over-Temperature protection

- OTP

A thermal shutdown is implemented to prevent dam-

ages due to excessive heat and power dissipation.

Typically the thermal shutdown threshold is 150

°C.

When the thermal shutdown is triggered the device

stops switching until the temperature falls below typi-

cally 135

°C. Then the device starts switching again.

Application Information

The selection of the inductor affects steady state op-

eration as well as transient behavior and loop stability.

There are three important electrical parameters which

need to be considered when choosing an inductor: the

value of inductor, DCR (copper wire resistance) and

the saturation current.

Choose an inductor that can handle the necessary

peak current without saturating, and ensure that the in-

ductor has a low DCR to minimise power losses. A

10

mH~22mH inductor should be a good choice for most

HT7939 applications. However, a more exact induc-

tance value can be calculated. A good rule for choosing

an inductor value is to allow the peak-to-peak ripple

current to be approximately 30~50% of the maximum

input current. Calculate the required inductance value

using the following equation:

current,

The output capacitor determines the steady state out-

put voltage ripple. The voltage ripple is related to the

capacitor

¢s capacitance and its ESR (Equivalent Se-

ries Resistance). A ceramic capacitor with a low ESR

value will provide the lowest voltage ripple and are

therefore recommended. Due to its low ESR, the ca-

pacitance value can be calculated by the equation:

A1

mF~10mF ceramic capacitor is suitable for most ap-

plication.

An input capacitor is required to supply the ripple cur-

rent to the inductor, while limiting noise at the input

source. A low ESR ceramic capacitors is required to

keep the noise at the IC to a minimum.

A 4.7

mF~10mF ceramic capacitor is suitable for most

application. This capacitor must be connected very

close to the VIN pin and inductor, with short traces for

good noise performance.

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