Datasheet

LED Driver with Integrated MOSFET
for MR16 and Other 12V AC Input Lamps
The IC has a control pin, REFI, to program the input
current. The IC regulates the average voltage on FB to
200mV if REFI is left open. A control voltage on REFI can
be used to control the input current. The average voltage
on FB is regulated to V
REFI
/6, where V
REFI
is the voltage
on REFI. The linear control of the FB voltage ceases once
the voltage on REFI exceeds 1.2V. Once the voltage
increases beyond 1.2V, the voltage on FB is regulated
to 200mV. REFI has an internal current source of 50FA
and the voltage on REFI can also be set with a resistor to
ground on REFI. This pin can also be used with an NTC
resistor on the pin to achieve thermal foldback.
The IC has a separate EXT pin that can be used to guar-
antee that there is a kick-start of current at turn-on for
low-input voltages for proper operation with electronic
transformers. EXT drives an external npn transistor.
Once the regulator MOSFET switches for the first time,
after the IN voltage has passed the 5.5V UVLO thresh-
old, EXT is pulled to ground and the external npn transis-
tor is turned off.
Internal Oscillator
The IC has an internal oscillator with a fixed switching
frequency of 300kHz.
Input Voltage (IN)
The IC is powered by the voltage on IN. The operating
voltage range on IN is from 6.5V to 48V. An internal
UVLO is set at 5.6V. Below 5.4V, there is no switching
of the internal power MOSFET and the gate driver for
the MOSFET is low. The typical hysteresis of the UVLO
threshold is 200mV. There is an internal LDO of 5V that is
used to power all the internal circuitry and the gate driver
for the internal switching MOSFET. An internal overvolt-
age protection on IN stops switching once the voltage on
IN exceeds 46V. The switching of the internal MOSFET
ceases once the rising voltage on IN exceeds 46V and
stays off until the voltage on IN drops by 1.2V from the
OVP threshold of 46V.
External BJT Driver (EXT)
EXT is a separate driver for an external npn transistor
that is used to drive an external resistive load when the
input voltage on IN is below the UVLO. This is useful
for operation with electronic transformers at low-input
voltages. The minimum current capability of this pin is
20mA.
Internal Switching MOSFET
The IC has an integrated switching MOSFET with a maxi-
mum R
DSON
of 0.2I at +125NC. The typical R
DSON
at
+25NC is 0.1I. This allows the IC to be used in a boost
LED driver for power levels up to 20W and for buck-
boost applications in MR16s for power levels up to 7W.
The maximum voltage rating of the MOSFET is 48V oper-
ation. The drain of the internal MOSFET is connected to
the DRAIN pin and the source of the internal MOSFET is
connected to the SOURCE pin.
Current Sense (FB)
The source of the internal MOSFET is connected to
SOURCE, so a current-sense resistor must be connected
between SOURCE and ground and the current informa-
tion is read by FB, so SOURCE must be connected to FB.
The set point of the current is determined by REFI. If REFI
is left open, the average current-sense voltage on FB is
regulated by the IC’s control loop to 200mV. There is a
separate peak-limit comparator that terminates switching
every cycle if the voltage on FB exceeds 650mV. This
comparator has a leading-edge blanking time of 50ns.
Control Loop
The IC uses an average current-mode control scheme
to regulate the input current (Figure 1). The control loop
regulates the average voltage on FB. An internal RC filter
removes current spikes appearing on this pin. Additional
filtering can be added if necessary. The current-regula-
tion loop consists of the current-sense resistor RCS, the
RC filter shown in Figure 1, the transconductance error
amplifier (g
m
), an oscillator providing the 300kHz ramp,
the control voltage on the positive input of the g
m
ampli-
fier, and the PWM comparator (PWMC).
Maxim Integrated
7
MAX16840
Maxim Integrated
7