PAM99700 Datasheet, PDF (7/12 Pages) Diodes Incorporated – UNIVERSAL HIGH BRIGHTNESS LED DRIVER

A Product Line of
Diodes Incorporated
PAM99700
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
inductor starts ramping up. This current flows through the external sense resistor R_CS and produces a ramp voltage at the CS pin. 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
inductor, the current sense resistor R_CS can be set using:
Rcs = 0.25V (or V_LD)/1.15 * I_LED(A)
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
pin, the PAM99700 maintains a constant 12V at the V_DD pin. This voltage is used to power the IC and any external resistor dividers needed to
control the IC. The V_DD pin must be bypassed by a low ESR capacitor to provide a low impedance path for the high frequency current of the
output gate driver.
ThePAM99700 can al so be operated by supplying a voltage at the V_DD pin greater than the internally regulated voltage. This will turn off the
internal linear regulator of the IC and the PAM99700 will operate directly off the voltage supplied at the V_DD pin. Please note that this external
voltage at the V_DD pin should not exceed 15V.
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
the IC. For example, if an 8-pin (junction to ambient thermal resistance R_θJ-A = 115°C/W) PAM99700 draws about I_IN = 2mA from the VIN pin,
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:
VIN(MAX)
=
ΔT
RθJ−A
•
1
IIN
= ^100°C • ¹
115°C / W 2mA
= 435V
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).
IIN ^{≈ 350μA +} Q_G ^* f s
In the above equation, f_S is the switching frequency and Q_G is the gate charge of the external FET (which can be obtained from the datasheet of
the FET).
PAM99700
Document number: DSxxxxx Rev. 1 - 1
7 of 12
www.diodes.com
October 2012
© Diodes Incorporated