Zxld1371, Applications information – Diodes ZXLD1371 User Manual
Page 21
ZXLD1371
ZXLD1371
Document number: DS35436 Rev. 1 - 2
21 of 42
February 2012
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Applications Information
(cont.)
b) Boost and Buck-Boost modes – the most simple boost/buck-boost circuit is shown in Figure 3
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:
1) the load voltage VLEDS plus the forward
voltage of D1 in Boost configuration,
or
2) the load voltage VLEDS plus the forward voltage
of D1 plus VIN in Buck-boost configuration.
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.
Figure 3. Boost and Buck-boost configuration
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. Note that in
reality, a load capacitor C
OUT
is used, so that the LED
current waveform shown is smoothed.
The average current in the sense resistor and coil, I
RS
, is
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
LED
, is always less than
I
RS
. The feedback control loop adjusts the switch duty
cycle, D, to achieve a set point at the sense resistor. This
controls I
RS
. During the interval t
OFF
, the coil current flows
through D1 and the LED load. During t
ON
, the coil current
flows through Q1, not the LEDs. Therefore the set point is
modified by D using a gating function to control I
LED
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
GI1
and R
GI2
.
(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.
GI_ADJ
=
RGI1
RGI1 +RGI2
Equation 2
(Boost and Buck-boost modes)
The control loop sets the duty cycle so that the sense
resistor current is
I
RS
=
0.225
R
S
GI_ADJ
1-D
V
ADJ
V
REF
Equation 3
(Boost and Buck-boost modes)
Figure 4. Operating waveforms (Boost and
Buck-boost modes)
I
RS
equals the coil current. The coil is connected only to the switch and the schottky diode. The schottky diode passes the
LED current. Therefore the average LED current is the coil current multiplied by the schottky diode duty cycle, 1-D.