MXHV9910

R01

www.clare.com

7

It is a good practice to select a power rating that is at

least twice the calculated value. This will give proper

margins, and make the design more reliable.

2.2.3 Current Sense Blanking

The MXHV9910 has an internal current-sense

blanking circuit. When the power MOSFET is turned

on, the external inductor can cause an undesired

spike at the current sense pin, CS, initiating a

premature termination of the gate pulse. To avoid this

condition, a typical 400ns internal leading edge

blanking time is implemented. This internal feature

eliminates the need for external RC filtering, thus

simplifying the design. During the current sense

blanking time, the current limit comparator is disabled,

preventing the gate-drive circuit from terminating the

gate-drive signal.

2.2.4 Enable/Disable

driver. Connecting PWMD to GND disables the gate

driver and sets the device into the shut-down mode. In

the shut-down mode, the gate output drive is disabled

while all other functions remain active. The maximum

quiescent current in the shut-down mode is 0.6mA.

2.2.5 Oscillator

The MXHV9910 operates in a constant frequency

mode. Setting the oscillator frequency is achieved by

In general, switching frequency selection is based on

the inductor size, controller power dissipation, and the

input filter capacitor.

is between 30kHz and 120kHz. This operating range

gives designers a reasonable compromise between

switching losses and inductor size. The internal RC

oscillator has a frequency accuracy of ±20%. Figure 4

Figure 4 Resistor Selection

2.2.6 Inductor Design

The inductor value is determined based on LED ripple

current, maximum on-time, the forward voltage drop of

all LEDs in a string at the desired current, and the

minimum input voltage, which is based on design

requirements. The maximum on-time is determined by

the duty cycle and switching frequency. The maximum

duty cycle is given by:

Where:

average LED current.

The maximum duty cycle must be restricted to less

than 50% in order to prevent sub-harmonic oscillations

and open loop instability.

The converter maximum ON-time is given by:

oscillator.

The inductor value for the given ripple is:

0

50

100

150

200

250

0

200

400

600

800

1000

1200

R

Oscillator Frequency, f

(T

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

=

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

=

–

() t

ONmax

×

×

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

=