Diodes PAM99700 User Manual

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Description

The PAM99700 is an open loop current mode control LED driver IC.

The PAM99700 can be programmed to operate in either a constant

frequency or constant off-time mode. It includes a 12 - 500V regulator

which allows it to work from a wide range of input voltages without the

need for an external low voltage supply. The PAM99700 includes a

PWM dimming input that can accept an external control signal with a

duty ratio of 0 - 100% and a frequency of up to a few kilohertz. It also

includes a 0 - 250mV linear dimming input which can be used for

linear dimming of the LED current.

The PAM99700 is ideally suited for buck LED drivers. Since the

PAM99700 operates in open loop current mode control, the controller

achieves good output current regulation without the need for any loop

compensation. PWM dimming response is limited only by the rate of

rise and fall of the inductor current, enabling very fast rise and fall

times. The PAM99700 requires only three external components (apart

from the power stage) to produce a controlled LED current making it

an ideal solution for low cost LED drivers.

Features

UNIVERSAL HIGH BRIGHTNESS LED DRIVER

Pin Assignments

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PAM99700

• Switch Mode Controller for Single Switch LED Drivers

• Open Loop Peak Current Controller

• Constant Frequency or Constant Off-Time Operation

• Linear and PWM Dimming Capability

• Requires Few External Components for Operation

• Application From a Few mA to More than 1A Output

PAM99700

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Applications

• DC/DC or AC/DC LED Driver Applications

• RGB Backlighting LED Driver

• Back Lighting of Flat Panel Displays

• General Purpose Constant Current Source

• Signage and Decorative LED Lighting

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Typical Applications Circuit

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PAM99700

Pin Descriptions

Name

VIN 1 1

CS 2 4

GND 3 5

GATE 4 8

PWMD 5 9

VDD 6 12

LD 7 13

RT 8 14

NC —

PAM99700

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Pin Number

SOP-8 SOP-16

This pin is the input of a 12V-500V regulator.

This pin is the current sense pin used to sense the FET current by means of an external sense resistor. When this pin exceeds the lower of either the internal 250mV or the voltage at the LD pin, the gate output goes low.

Ground return for all internal circuitry. This pin must be electrically connected to the ground of the power train.

This pin is the output gate driver for an external N-Channel power MOSFET.

This is the PWM dimming input of the IC. When this pin is pulled to GND, the gate driver is turned off. When the pin is pulled high, the gate driver operates normally.

This is the power supply pin for all internal circuits. It must be bypassed with a low ESR capacitor to GND (~ 10μF).

This pin is the linear dimming input an d sets the current sense threshold as long as the voltage at the pin is less than 250mV (typ).

This pin sets the oscillator frequency. When a resistor is connected between RT and GND, the PAM99700 operates in constant frequency mode. When the resistor is connected between RT and GATE, the IC operates in constant off-time mode.

2, 3, 6, 7,

10, 11, 15, 16

Not connected.

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Function

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Functional Block Diagram

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PAM99700

Absolute Maximum Ratings (@T

These are stress ratings only and functional operation is not implied. Exposure to absolute maximum ratings for prolonged time periods may affect device reliability. All voltages are with respect to ground.

Parameter Rating Unit

Maximum Junction Temperature 150

Storage Temperature -65 to +150

Soldering Temperature 300, 5sec

Recommended Operating Conditions (@T

Parameter Rating Unit

Supply Voltage 12 to 500 V

Ambient Temperature Range -40 to +85 °C

Junction Temperature Range -40 to +125 °C

= +25°C, unless otherwise specified.)

°C

= +25°C, unless otherwise specified.)

Thermal Information

Parameter Package Symbol Max Unit

Thermal Resistance (Junction to Ambient)

PAM99700

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SOP-8

SOP-16 110

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115

°C/W

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Electrical Characteristics (@T

Parameter Symbol Test Conditions Min Typ Max Units

Input Voltage Range

Shutdown Mode Supply Current

Internally Regulated Voltage

Current Sense Pul-In Threshold Voltage

Current Sense Blanking Interval

Gate Sourcing Current

Gate Sinking Current

Gate Output Rise Time

Gate Output Fall Time

Oscillator Frequency

EN Threshold High

EN Threshold Low

Over Temperature Protection OTP 160 °C

OTP Hysteresis OTH 50 °C

PAM99700

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= +25°C, unless otherwise specified.)

INDC

INSD

CS,TH

BLANK

SOURCE

SINK

RISE

TALL

OSC

DC Input Voltage

Pin PWMD to GND

VIN > 20V

250 mV

250 Ns

= 0V

GATE

= VDD

GATE

= 1nF

GATE

= 1nF

GATE

= 510kΩ

RT = 226kΩ

VIN = 12V to 500V

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12 500 V

0.1 mA

6 10 V

0.2 A

30 50 ns

37 41 49

74 92 110

kHz

2.4 V

1.0 V

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Typical Performance Characteristics

(@TA = +25°C, VIN = 60V, 1WLED, RT = 510KΩ, L = 5.2mH, unless otherwise specified.)

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Typical Performance Characteristics (cont.)

(@TA = +25°C, VIN = 110VAC, 1WLED, RT = 510KΩ, L = 5.2mH, RCS = 0.68Ω, unless otherwise specified.)

PAM99700

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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

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

Rcs = 0.25V (or V

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).

)/1.15 * I

can be set using:

(A)

LED

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

control the IC. The V

output gate driver.

ThePAM99700 can al so be operated by supplying a voltage at the V

internal linear regulator of the IC and the PAM99700 will operate directly off the voltage supplied at the V

voltage at the V

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

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:

pin must be bypassed by a low ESR capacitor to provide a low impedance path for the high frequency current of the

pin should not exceed 15V.

)MAX(IN

•

INAJ

−θ

C100

•

mA2

W/C115

V435=

pin. This voltage is used to power the IC and any external resistor dividers needed to

pin greater than the internally regulated voltage. This will turn off the

= 115°C/W) PAM99700 draws about IIN = 2mA from the VIN pin,

θJ-A

and produces a ramp voltage at the CS pin. The

pin. Please note that this external

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).

+μ≈

In the above equation, f

the FET).

A350

is the switching frequency and QG is the gate charge of the external FET (which can be obtained from the datasheet of

PAM99700

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Application Information (cont.)

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

these cases, an external RC filter needs to be added between the external sense resistor

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:

F+Ω=

21380

)kHz(

5.5)k(RT

and the CS pin.

(RCS)

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.

+Ω

1.44)k(RT

OFF

(µS)

1.21

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

some cases, it may not be possible to find the exact R

cases, an external voltage divider from the V

desired voltage across R

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.

To use the internal 250mV, the LD pin can be connected to V

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.

pin can be connected to the LD pin to obtain a voltage (less than 250mV) corresponding to the

value required to obtain the LED current when the internal 250mV is used. In these

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

to V

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.

PAM99700

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Ordering Information

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PAM99700

Marking Information

Part Number Package Type Standard Package

PAM99700AC SOP-8L 2500 Units/Tape&Reel

PAM99100BD SOP-16L 2500 Units/Tape&Reel

PAM99700

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Package Outline Dimensions (All dimensions in mm.)

SOP-8

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PAM99700

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Package Outline Dimensions (cont.) (All dimensions in mm.)

SOP-16

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PAM99700

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