HV9910B Ver la hoja de datos (PDF) - Unspecified

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HV9910B

Universal High Brightness LED Driver

Unspecified 

HV9910C

Application Information

The HV9910C is optimized to drive buck LED drivers using

open-loop peak current mode control. This method of control

enables fairly accurate LED current control without the need

for high side current sensing or the design of any closed loop

controllers. The IC uses very few external components and

enables both Linear and PWM dimming of the LED current.

A resistor connected to the RT pin programs the frequency

of operation (or the off-time). The oscillator produces pulses

at regular intervals. These pulses set the SR flip-flop in the

HV9910C which causes the GATE driver to turn on. The same

pulses also start the blanking timer which inhibits the reset

input of the SR flip flop and prevent false turn-offs due to the

turn-on spike. When the FET turns on, the current through

the inductor starts ramping up. This current flows through

the external sense resistor Rcs and produces a ramp voltage

at the CS pin. The comparators are constantly comparing

the CS pin voltage to both the voltage at the LD pin and

the internal 250mV. Once the blanking timer is complete, the

output of these comparators is allowed to reset the flip flop.

When the output of either one of the two comparators goes

high, the flip flop is reset and the GATE output goes low. The

GATE goes low until the SR flip flop is set by the oscillator.

Assuming a 30% ripple in the inductor, the current sense

resistor Rcs can be set using:

RCS =

0.25V (or VLD )

1.15 • ILED

Constant frequency peak current mode control has an

inherent disadvantage – at duty cycles greater than 0.5,

the control scheme goes into subharmonic oscillations.

To prevent this, an artificial slope is typically added to the

current sense waveform. This slope compensation scheme

will affect the accuracy of the LED current in the present

form. However, a constant off-time peak current control

scheme does not have this problem and can easily operate

at duty cycles greater than 0.5 and also gives inherent input

voltage rejection making the LED current almost insensitive

to input voltage variations. But, it leads to variable frequency

operation and the frequency range depends greatly on the

input and output voltage variation. HV9910C makes it easy

to switch between the two modes of operation by changing

one connection (see oscillator section).

Input Voltage Regulator

The HV9910C can be powered directly from its VIN pin and

can work from 15 - 450VDC at its VIN pin. When a voltage

is applied at the VIN pin, the HV9910C maintains a constant

7.5V at the VDD pin. This voltage is used to power the IC

and any external resistor dividers needed to control the IC.

The VDD pin must be bypassed by a low ESR capacitor to

provide a low impedance path for the high frequency current

of the output GATE driver.

The HV9910C can also be operated by supplying a voltage

at the VDD pin greater than the internally regulated voltage.

This will turn off the internal linear regulator of the IC and the

HV9910C will operate directly off the voltage supplied at the

VDD pin. Please note that this external voltage at the VDD

pin should not exceed 12V.

Although the VIN pin of the HV9910C is rated up to 450V, the

actual maximum voltage that can be applied is limited by the

power dissipation in the IC. For example, if an 8-lead SOIC

(junction to ambient thermal resistance Rθj-a = 101°C/W)

HV9910C draws about IIN = 2.0mA from the VIN pin, and

has a maximum allowable temperature rise of the junction

temperature limited to ΔT = 75°C, the maximum voltage at

the VIN pin would be:

ΔT 1

VIN(MAX) = Rθja ● IIN

75OC

1

=

●

101OC/W 2mA

= 371V

In these cases, to operate the HV9910C from higher input

voltages, a Zener diode can be added in series with the VIN

pin to divert some of the power loss from the HV9910C to

the Zener diode. In the above example, using a 100V zener

diode will allow the circuit to easily work up to 450V.

Note: The Zener diode will increase the minimum input

voltage required to turn on the HV9910C to 115V.

The input current drawn from the VIN pin is a sum of the

1.5mA (maximum) current drawn by the internal circuit and

the current drawn by the GATE driver (which in turn depends

on the switching frequency and the GATE charge of the

external FET).

IIN = 1.5mA + Qg • fs

In the above equation, fs is the switching frequency and

Qg is the GATE charge of the external FET (which can be

obtained from the datasheet of the FET).

Current Sense

The current sense input of the HV9910C goes to the non-

inverting inputs of two comparators. The inverting terminal

Doc.# DSFP-HV9910C

NR041813

Supertex inc.

4

www.supertex.com

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