HV9911NG 데이터 시트보기 (PDF) - Supertex Inc

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HV9911NG

Switch-mode LED Driver IC with High Current Accuracy

Supertex Inc 

HV9911

Reference

Current Sense

HV9911 includes a 2% accurate, 1.25V reference, which can

be used as the reference for the output current as well as

to set the switch current limit. This reference is also used

internally to set the over voltage protection threshold. The

reference is buffered so that it can deliver a maximum of

500µA external current to drive the external circuitry. The

reference should be bypassed with at least a 10nF low ESR

capacitor.

Oscillator

The oscillator can be set in two ways. Connecting the

oscillator resistor between the RT and GATE pins will

program the off-time. Connecting the resistor between RT

and GND will program the time period.

In both cases, resistor RT sets the current, which charges

an internal oscillator capacitor. The capacitor voltage ramps

up linearly and when the voltage increases beyond the

internal set voltage, a comparator triggers the SET input of

the internal SR flip-flop. This starts the next switching cycle.

The time period of the oscillator can be computed as:

Ts ≈ RT × 11 pF

Slope Compensation

For converters operating in the constant frequency mode,

slope compensation becomes necessary to ensure stability

of the peak current mode controller, if the operating duty

cycle is greater than 0.5. Choosing a slope compensation

which is one half of the down slope of the inductor current

ensures that the converter will be stable for all duty cycles.

Slope compensation can be programmed by two resistors

RSLOPE and RSC. Assuming a down slope of DS (A/µs) for the

inductor current, the slope compensation resistors can be

computed as:

Rslope

=

DS

10 × RSC

× 106 × Ts

×

Rcs

The current sense input of the HV9911 includes a built in

100ns (minimum) blanking time to prevent spurious turn off

due to the initial current spike when the FET turns on.

The HV9911 includes two high-speed comparators – one is

used during normal operation and the other is used to limit

the maximum input current during input under voltage or

overload conditions.

The IC includes an internal resistor divider network, which

steps down the voltage at the COMP pin by a factor of

15. This stepped-down voltage is given to one of the

comparators as the current reference. The reference to the

other comparator, which acts to limit the maximum inductor

current, is given externally.

It is recommended that the sense resistor RCS be chosen so

as to provide about 250mV current sense signal.

Current Limit

Current limit has to be set by a resistor divider from the

1.25V reference available on the IC. Assuming a maximum

operating inductor current ipk (including the ripple current),

the maximum voltage at the CLIM pin can be set as:

VCLIM

≥ 1.2 × I pk × Rcs

+ 5 × Rsc

Rslope

× 0.9

Note that this equation assumes a current limit at 120%

of the maximum input current. Also, if VCLIM is greater than

450mV, the saturation of the internal opamp will determine

the limit on the input current rather than the CLIM pin. In such

a case, the sense resistor RCS should be reduced till VCLIM

reduces below 450mV.

It is recommended that no capacitor be connected between

CLIM and GND. If necessary, the capacitor value must be

chosen to be less than 1000pF.

FAULT protection

A typical value for Rsc is 499Ω.

Note: The maximum current that can be sourced out of the

SC pin is limited to 100µA. This limits the minimum value

of the RSLOPE resistor to 25kΩ. If the equation for slope

compensation produces a value of RSLOPE less than this

value, then RSC would have to be reduced accordingly. It is

recommended that RSLOPE be chosen in the range of 25kΩ

- 50kΩ.

The HV9911 has built-in output over-voltage protection

and output short circuit protection. Both protection features

are latched, which means that the power to the IC must

be recycled to reset the IC. The IC also includes a FAULT

pin which goes low during any fault condition. At startup, a

monoshot circuit, (triggered by the POR circuit), resets an

internal flip-flop which causes FAULT to go high, and remains

high during normal operation. This also allows the gate drive

to function normally. This pin can be used to drive an external

disconnected switch (Q2 in the Typical Boost Application

Circuit on pg.1), which will disconnect the load during a

fault condition. This disconnect switch is very important in a

boost converter, as turning off the switching FET (Q1) during

NR011206

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