PAM99700

Document number: DSxxxxx Rev. 1 - 1

7 of 12

www.diodes.com

October 2012

© Diodes Incorporated

PAM99700

A Product Line of

Diodes Incorporated

Application Information

The PAM99700 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 fl ip- flop in the PAM99700 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

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

Constant frequency peak current mode controlhas 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 then 0.5 and also gives inherent input volt-age 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. PAM99700 makes it easy to switch between the two modes of operation by changing one connection (see oscillator

section).

Input Voltage Regulator

The PAM99700 can be powered directly from its VIN pin and can work from 12 - 500V DC at its VIN pin. When a voltage is applied at the VIN

output gate driver.

Although the VIN pin of the PAM99700 is rated up to 500V, the actual maximum voltage that can be applied is limited by the power dissipation in

and has a maximum allowable temperature rise of the junction temperature limited to about ΔT = 100°C, the maximum voltage at the VIN pin

would be:

I

1

R

T

V

IN

A

J

)

MAX

(

IN

•

Δ

=

−

θ

mA

2

1

W

/

C

115

C

100

•

°

°

=

V

435

=

In these cases, to operate the PAM99700 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 PAM99700 to the Zener diode. In the above example, using a 100V zener diode will allow the circuit to easily work up to

500V.

The input current drawn from the VIN pin is a sum of the 1.0mA 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).

f

*

Q

A

350

I

s

G

IN

+

μ

≈

the FET).