HM4523 datasheet(4/8 Pages) HMSEMI

4 / 8 page

Functional Description

CV/CC Loop Regulation

As seen in Functional Block Diagram, the

HM4523 is a

peak current mode pulse width modulation (PWM)

converter with CC and CV control. The converter

operates as follows:

A switching cycle starts when the rising edge of the

Oscillator clock output causes the High-Side Power

Switch to turn on and the Low-Side Power Switch to turn

off. With the SW side of the inductor now connected to

VIN, the inductor current ramps up to store energy in the

magnetic field. The inductor current level is measured by

the Current Sense Amplifier and added to the Oscillator

ramp signal. If the resulting summation is higher than the

COMP voltage, the output of the PWM Comparator goes

high. When this happens or when Oscillator clock output

goes low, the High-Side Power Switch turns off.

At this point, the SW side of the inductor swings to a

diode voltage below ground, causing the inductor current

to decrease and magnetic energy to be transferred to

output. This state continues until the cycle starts again.

The High-Side Power Switch is driven by logic using BS

as the positive rail. This pin is charged to VSW + 5V when

the Low-Side Power Switch turns on. The COMP voltage

is the integration of the error between FB input and the

internal 0.808V reference. If FB is lower than the

reference voltage, COMP tends to go higher to increase

current to the output. Output current will increase until it

reaches the CC limit set by the ISET resistor. At this

point, the device will transition from regulating output

voltage to regulating output current, and the output

voltage will drop with increasing load.

The Oscillator normally switches at 230kHz. However, if

FB voltage is less than 0.6V, then the switching

frequency decreases to 50kHz gradually.

Enable Pin

The

HM4523 has an enable input EN for turning the IC on

or off. The EN pin contains a precision 1.6V comparator

with 125mV hysteresis and a 10μA pull-up current source.

The comparator can be used with a resistor divider from

VIN to program a startup voltage higher than the normal

UVLO value. It can be used with a resistor divider from

VOUT to disable charging of a deeply discharged battery,

or it can be used with a resistor divider containing a

thermistor to provide a temperature-dependent shutoff

protection for over temperature battery. The thermistor

should be thermally coupled to the battery pack for this

usage.

If left floating, the EN pin will be pulled up to roughly 5V

by the internal 10μA current source. It can be driven from

standard logic signals greater than 1.6V, or driven with

open-drain logic to provide digital on/off control.

Thermal Shutdown

The

HM4523 disables switching when its junction

temperature exceeds 175°C and resumes when the

temperature has dropped by 40°C.

APPLICATIONS INFORMATION

Output Voltage Setting

Figure 4:

Output Voltage Setting

Vout

RFB1

RFB2

Figure 4 shows the connections for setting the output

voltage. Select the proper ratio of the two feedback

resistors RFB1 and RFB2 based on the output voltage.

Adding a capacitor in parallel with RFB1 helps the system

stability. Typically, use RFB2 ≈10kΩ and determine RFB1

from the following equation:

















808

OUT

CC Current Setting

HM4523 constant current value is set by a resistor

connected between the ISET pin and GND. The CC

output current is approximating linearly proportional to

the current flowing out of the ISET pin. The voltage at

ISET is roughly 1V and the current gain from ISET to

output is roughly 27500 (27.5mA/1μA). To determine the

proper resistor for a desired current, please refer to Figure

5 below.

Figure 5:

Curve for Programming Output CC Current

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1012141618202224

RISET(kΩ)

Output Current vs RISET

VIN=24V,Vout=4V

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