ZXLD1371EST16TC Diodes Inc. / Zetex, ZXLD1371EST16TC Datasheet - Page 21

• Manufacturer Part Number: ZXLD1371EST16TC
• Description: MOSFET LED Controller TSSOP TSSOP-16EP T&R 2.5K
• Manufacturer: Diodes Inc. / Zetex

Specifications of ZXLD1371EST16TC

• Rohs: yes

ZXLD1371
Document number: DS35436 Rev. 1 - 2
Applications Information
b) Boost and Buck-Boost modes – the most simple boost/buck-boost circuit is shown in Figure 3
I
LED current. Therefore the average LED current is the coil current multiplied by the schottky diode duty cycle, 1-D.
RS
Control in Boost and Buck-boost mode is achieved by
sensing the coil current in the series resistor Rs, connected
between the two inputs of a current monitor within the
control loop block. An output from the control loop drives
the input of a comparator which drives the gate of the
external NMOS switch transistor Q1 via the internal Gate
Driver. When the switch is on, the drain voltage of Q1 is
near zero. Current flows from VIN, via Rs, coil and switch
to ground. This current ramps up until an upper threshold
value is reached (see Figure 4). At this point GATE goes
low, the switch is turned off and the drain voltage increases
to either:
or
Current flows via Rs, coil, D1 and LED back to VIN (Buck-
boost mode), or GND (Boost mode). When the coil current
has ramped down to a lower threshold value, GATE goes
high, the switch is turned on again and the cycle of events
repeats, resulting in continuous oscillation.
The feeback loop adjusts the NMOS switch duty cycle to
stabilize the LED current in response to changes in external
conditions, including input voltage and load voltage. Loop
compensation is achieved by a single external capacitor
C2, connected between SHP and SGND.
reality, a load capacitor C
current waveform shown is smoothed.
The average current in the sense resistor and coil, I
equal to the average of the maximum and minimum
threshold currents and the ripple current (hysteresis) is
equal to the difference between the thresholds.
The average current in the LED, I
I
cycle, D, to achieve a set point at the sense resistor. This
controls I
through D1 and the LED load. During t
flows through Q1, not the LEDs. Therefore the set point is
modified by D using a gating function to control I
indirectly. In order to compensate internally for the effect of
the gating function, a control factor, GI_ADJ is used.
GI_ADJ is set by a pair of external resistors, R
(Figure 3.) This allows the sense voltage to be adjusted to
an optimum level for power efficiency without significant
error in the LED controlled current.
The control loop sets the duty cycle so that the sense
resistor current is
RS
equals the coil current. The coil is connected only to the switch and the schottky diode. The schottky diode passes the
.
The feedback control loop adjusts the switch duty
2) the load voltage VLEDS plus the forward voltage
1) the load voltage VLEDS plus the forward
RS
GI_ADJ =
I
RS
of D1 plus VIN in Buck-boost configuration.
. During the interval t
voltage of D1 in Boost configuration,
=
(Boost and Buck-boost modes)
(Boost and Buck-boost modes)
0.225
R
S
RGI1 +RGI2
RGI1
GI_ADJ
1-D
OUT
is used, so that the LED
(cont.)
OFF
V
V
LED
ADJ
REF
, the coil current flows
, is always less than
ON
, the coil current
Equation 3
Equation 2
Note that in
GI1
and R
www.diodes.com
RS
21 of 42
, is
LED
GI2
.
Figure 3. Boost and Buck-boost configuration
Figure 4. Operating waveforms (Boost and
Diodes Incorporated
Buck-boost modes)
A Product Line of
© Diodes Incorporated
February 2012
ZXLD1371