HV9912 datasheet(8/20 Pages) MICROCHIP

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HV9912

DS20005583A-page 8

 2016 Microchip Technology Inc.

3.0

DETAILED DESCRIPTION

3.1

Power Topology

The HV9912 is a Switch-mode converter LED driver

designed to control a Continuous Conduction mode

buck or boost in a Constant Frequency or Constant

Off-time mode. The IC includes an internal linear

regulator, which operates from input voltages up to

90V, eliminating the need for an external power supply

for the IC. The IC includes features typically required in

LED drivers, such as open LED protection, output

short-circuit protection, linear and PWM dimming,

programmable input current limiting and accurate

control of the LED current. A high-current gate drive

output enables the controller to be used in high-power

converters.

The HV9912 is an enhanced version of the HV9911

with hysteretic overvoltage protection and Hiccup

mode short-circuit protection. The IC includes a

blanking network controlled by the PWMD input to

prevent the short-circuit protection from triggering

prematurely during PWM dimming due to the parasitic

capacitance of the LED string. It also allows the IREF

pin to be pulled all the way down to GND without

triggering

the

short-circuit

protection.

pin-compatible replacement for the HV9911.

3.2

Linear Regulator

The HV9912 can be powered directly from its VIN pin

that withstands a voltage of up to 90V. When a voltage

is applied to the VIN pin, the HV9912 tries to maintain a

constant 7.75V (typical) at the VDD pin. The regulator

also has a built-in undervoltage lockout which shuts off

the IC if the voltage at the VDD pin falls below the UVLO

threshold.

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

(≥0.1 µF) to provide a low-impedance path for the

high-frequency current of the output gate driver.

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

the 1.5 mA 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. See Equation 3-1.

EQUATION 3-1:

IIN

1.5mA

QG fS





In the above equation, fS is the switching frequency,

and QG is the external FET’s gate charge, which can be

obtained from the data sheet of the FET.

3.3

Minimum Input Voltage at VIN Pin

The minimum input voltage at which the converter will

start and stop depends on the minimum voltage drop

required for the linear regulator. The internal linear

regulator will control the voltage at the VDD pin when

VIN is between 9V and 90V. However, when VIN is less

than 9V, the converter will still function as long as VDD

is greater than the undervoltage lockout. Thus, the

converter might be able to start at input voltages lower

than 9V. The start/stop voltages at the VIN pin can be

determined using the minimum voltage drop across the

linear regulator as a function of the current drawn. This

data is shown in Figure 3-1 for ambient temperatures of

25°C and 85°C.

Assume an ambient temperature of 85°C. Provided

that the IC is driving a 15 nC gate charge FET at

200 kHz, the total input current is estimated to be

4.5 mA when Equation 3-1 is used. At this input

current, the minimum voltage drop from Figure 3-1

would be around VDROP = 1.25V. However, before the

IC starts switching, the current drawn would have been

1.5 mA. At this current level, the voltage drop would be

approximately VDROP1 = 0.3V. Thus, the start/stop VIN

voltages could be computed as demonstrated in

Equation 3-2 and Equation 3-3 below:

EQUATION 3-2:

VIN START



UVLOMAX VDROP1

7V 0.3V

7.3V

EQUATION 3-3:

VIN STOP



UVLOMAX UVLO

–

VDROP

7V 0.5V

–1.25V

7.75V

Minimum Voltage Drop vs. I

0.5

1.5

2.5

IN (mA)

A = 25

A = 85

FIGURE 3-1:

Headroom vs. Input Current.

In this case, the gate driver draws too much current and

VINSTART is less than VINSTOP. When this happens, the

IC will oscillate between ON and OFF if the input

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