HV9961 datasheet(6/20 Pages) MICROCHIP

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HV9961

DS20005588B-page 6

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3.0

APPLICATION INFORMATION

3.1

General Description

Peak current control (as in HV9910B) is the simplest

and the most economical way to regulate a buck

converter's output current. However, it suffers accuracy

and

regulation

problems

that

arise

from

peak-to-average current error, contributed by the

current ripple in the output inductor and the

propagation delay in the current sense comparator.

The full inductor current signal is unavailable for direct

switch current sensing across the sense resistor at the

ground path in this low-side switch buck converter

when the control switch is at the ground potential

because the switch is turned off. While it is very simple

to detect the peak current in the switch, controlling the

average inductor current is usually implemented by

level translating the sense signal from +VIN. Although

this is practical for a relatively low-input voltage, VIN,

this type of average-current control may become

excessively complex and expensive in the offline AC or

other high-voltage DC applications.

The HV9961 uses a proprietary control scheme that

allows fast and accurate control of the average current

in the buck inductor by sensing the switch current only.

No compensation of the current control loop is

required. The output LED current’s response to PWMD

input is similar to that of the HV9910B. The effect of

inductor current ripple amplitude on this control

scheme is insignificant. Therefore, the LED current is

independent of the variation in inductance, switching

frequency or output voltage. Constant off-time control

of the buck converter is used for stability and improving

the LED current regulation over a wide range of input

voltages. Unlike HV9910B, the HV9961 does not

support Constant Frequency mode.

3.2

Off Timer

The timing resistor connected between RT and GND

determines the off-time of the gate driver. Wiring this

resistor between RT and Gate as with HV9910B is no

longer supported. Refer to Equation 3-1 for the

computation of the gate output’s off-time.

EQUATION 3-1:

3.3

Average-Current Control

Feedback and Output Short-circuit

Protection

The current through the switching Metal-oxide

Semiconductor Field-effect Transistor (MOSFET)

source is averaged and used to give constant-current

feedback. This current is detected with a sense resistor

at the CS pin. The feedback operates in a fast

Open-loop mode. No compensation is required. Output

current is programmed as seen in Equation 3-2.

EQUATION 3-2:

If the voltage at the LD input is less than 1.5V, the

output

current

computed

specified

Equation 3-3.

EQUATION 3-3:

The above equations are only valid for continuous

conduction of the output inductor. It is good design

practice to choose the inductance of the inductor such

that the peak-to-peak inductor current is 30% to 40% of

the average DC full-load current. Hence, the

recommended inductance can be calculated as shown

in Equation 3-4.

EQUATION 3-4:

The duty-cycle range of the current control feedback is

limited to D ≤ 0.75. A reduction in the LED current may

occur when the desired LED string voltage VO is

greater than 75% of the input voltage VIN of the

HV9961 LED driver.

Reducing the targeted output LED string voltage VO

below VO(MIN) = VIN x DMIN, where DMIN = 1 µs/(TOFF

+1 µs), may also result in the loss of regulation of the

LED current. This condition, however, causes an

increase in the LED current and can potentially trip the

short-circuit protection comparator.

The typical output characteristic of the HV9961 LED

driver is shown in Figure 3-1. The corresponding

HV9910B characteristic is given for the comparison.

TOFF s



RT k



-------------------0.3

within the range of 30 kΩ ≤ RT ≤ 1 MΩ

ILED

0.275V

RCS

-----------------

When the voltage at the LD input VLD ≥ 1.5V

ILED

VLD 0.185



RCS

------------------------------

When the voltage at the LD input 0.2V ≤ VLD < 1.5V

VOMAX



TOFF



0.4 IO



-----------------------------------------

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