PAM99700

Document number: DSxxxxx Rev. 1 - 1

8 of 12

www.diodes.com

October 2012

© Diodes Incorporated

PAM99700

A Product Line of

Diodes Incorporated

Current Sense

The current sense input of the PAM99700 goes to the non-inverting inputs of two comparators. The inverting terminal of one comparator is tied

to an internal 250mV reference whereas the inverting terminal of the other comparator is connected to the LD pin. The outputs of both these

comparators are fed into an OR gate and the output of the OR gate is fed into the reset pin of the flip-flop. Thus, the comparator which has the

lowest voltage at the inverting terminal determines when the GATE output is turned off.

The outputs of the comparators also include a 150-280ns blanking time which prevents spurious turn-offs of the external FET due to the turn-on

spike normally present in peak current mode control. In rare cases, this internal blanking might not be enough to filter out the turn-on spike. In

Please note that the comparators are fast (with a typical 80ns response time). Hence these comparators are more susceptible to be triggered by

noise than the comparators of the PAM99700. A proper layout minimizing external inductances will prevent false triggering of these comparators.

Oscillator

The oscillator in thePAM99700 is controlled by a single resistor connected at the RT pin. The equation governing the oscillator frequency is given

by:

)

kHz

(

5

.

5

)

k

(

RT

21380

F

+

Ω

=

If the resistor is connected between RT and GND, PAM99700 operates in a constant frequency mode and the above equation determines the

time-period. If the resistor is connected between RT and GATE, the PAM99700 operates in a constant off-time mode and the below equation

determines the off-time.

1

.

21

1

.

44

)

k

(

RT

+

Ω

=

(µS)

GATE Output

The gate output of the PAM99700 is used to drive and external FET. It is recommended that the gate charge of the external FET be less than

25nC for switching frequencies 100kHz and less than 15nC for switching frequencies >100kHz.

Linear Dimming

The Linear Dimming pin is used to control the LED current. There are two cases when it may be necessary to use the Linear Dimming pin. In

Linear dimming may be desired to adjust the current level to reduce the intensity of the LEDs. In these cases, an external 0-250mV voltage can

be connected to the LD pin to adjust the LED current during operation.

Note: Although the LD pin can be pulled to GND, the output current will not go to zero. This is due to the presence of a minimum on-time (which

is equal to the sum of the blanking time and the delay to output time) which is about 450ns. This will cause the FET to be on for a minimum of

450ns and thus the LED current when LD = GND will not be zero. This current is also dependent on the input voltage, inductance value, forward

voltage of the LEDs and circuit parasitics. To get zero LED current, the PWMD pin has to be used.

PWM Dimming

PWM Dimming can be achieved by driving the PWMD pin with a low frequency square wave signal. When the PWM signal is zero, the gate

driver is turned off and when the PWMD signal if high, the gate driver is enabled. Since the PWMD signal does not turn off the other parts of the

IC, the response of the PAM99700 to the PWMD signal is almost instantaneous. The rate of rise and fall of the LED current is thus determined

solely by the rise and fall times of the inductor current.To disable PWM dimming and enable the PAM99700 permanently, connect the PWMD pin

Thermal Protection

When the die temperature exceeds +160°C, a reset occurs and the reset remains until the temperature decrease to +110°C, at which time the

circuit can be restarted.When the die temperature exceeds +160°C, a reset occurs and the reset remains until the temperature decrease to

+110°C, at which time the circuit can be restarted.