Diodes ZXLD1370 - Manual (Page 15)

ZXLD1370
Document number: DS32165 Rev. 5 - 2
15 of 39
www.diodes.com
September 2012
© Diodes Incorporated
ZXLD1370
A Product Line of
Diodes Incorporated
Application Information
(cont.)
b) Boost and Buck-Boost modes – the most simple boost/buck-boost circuit is shown in Figure 23
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 24). 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. 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 23.)
This allows the sense voltage to be adjusted to an optimum level for power
efficiency without significant error in the LED controlled current.
⎟
⎠
⎞
⎜
⎝
⎛
+
=
2 RGI 1 RGI
1 RGI
ADJ _ GI
Equation 2 (Boost and Buck-boost modes)
The control loop sets the duty cycle so that the sense resistor current is
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⎟
⎠
⎞
⎜
⎝
⎛
−
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
V
V
D 1
ADJ _ GI
R
225 . 0
R
REF
ADJ
S
S
Equation 3 (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.
Figure 23 Boost and Buck-Boost Configuration
Figure 24 Operating Waveforms
(Boost and Buck-boost modes)