lx1668 Microsemi Corporation, lx1668 Datasheet - Page 7

• Manufacturer Part Number: lx1668
• Description: Programmable Multiple Output Dc Dc Controller
• Manufacturer: Microsemi Corporation

Copyright © 1999
Rev. 1.0
SWITCHER OUTPUT VOLTAGE REGULATION
Refer to the IC Block Diagram and the Product Highlight circuit.
When the top MOSFET turns ON, the inductor current increases.
The voltage at V
capacitor and the current-sensing resistor. When the V
voltage reaches the threshold voltage of the error comparator,
V
PWM latch is reset. Consequently, the top MOSFET turns OFF
and the bottom (synchronous) MOSFET turns ON. The off-time
control block controls the off-time of the top MOSFET. During
the off-time, the inductor current and the V
decrease. As the off-time finishes, the synchronous MOSFET
turns OFF and the top MOSFET turns ON again, repeating the
previous cycle. A break-before-make circuit prevents simulta-
neous conduction of the two MOSFET’s.
response of the output voltage, as shown in Figure 4 below.
changes allows a greater output voltage excursion during a fast
step-load transient and requires fewer output capacitors to meet
the transient-response specification.
SET
4/99
The 40mV offset to the set voltage enhances the transient
This adaptive positioning of the output voltage as the load
(the DAC output set-point voltage) plus 40mV offset, the
The peak voltage at the V
voltage and its average is the peak voltage minus the ripple
voltage at V
The output voltage is the voltage at the V
voltage drop across the current sensing resistor (I * R
At light loads, the voltage drop across the sensing resistor
is small; hence, the output voltage is approximately the
voltage at the V
set voltage, V
At heavy loads, larger current flows in the sense resistor,
therefore, the voltage drop is higher and the output voltage
is lower.
of the load current step and
Dynamic voltage tolerance
mainly due to the product
(ESL effects are ignored)
ESR of the capacitors.
V
Initial voltage drop is
DYN
voltage, V
Nominal set-point
Adaptive voltage
positioning offset
(100mV for 2µs)
FB
FB
V
pin increases due to the ESR of the output
SET
OFFSET
V = I * ESR
pin.
FB
).
pin (approximately 40mV higher than the
SET
(40mV)
(2.0V)
FB
P
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
pin is 40mV higher than the set
ROGRAMMABLE
P
L
FIGURE 4 — Adaptive Voltage Positioning
R O D U C T I O N
OUT
= 2.5µH, C
T H E O R Y O F O P E R A T I O N
FB
FB
pin minus the
pin voltage
M
U LTIPLE
OUT
= 6x1500µF Sanyo MV-GX, R
FB
SENSE
pin
).
O
UTPUT
D
POWER UP and INITIALIZATION
At power up, the LX1668 monitors the supply voltage to both the
+5V and the +12V pins (there is no special requirement for the
sequence of the two supplies). Before both supplies reach their
under-voltage lock-out (UVLO) thresholds, the soft-start (SS) pin
is held low to prevent soft-start from beginning; the off-time
control is disabled and the top MOSFET is kept OFF. The two
LDO regulators are designed not to over-voltage regardless of
supply voltages. After both supplies pass the UVLO thresholds,
the circuit begins soft-start.
SOFT-START
Once the supplies are above the UVLO threshold, the soft-start
capacitor begins to be charged up by the set voltage (DAC
output) through a 20k
at the SS pin rises as a simple RC circuit. The SS pin plus a 40mV
offset is connected to the error comparator’s non-inverting input
that controls the output peak voltage. The output voltage will
follow the SS pin voltage if sufficient charging current is provided
to the output capacitor.
beginning and slower at the end of the soft-start interval. Thus,
the required charging current into the output capacitor is less at
the end of the soft-start interval so decreasing the possibility of
an over-current. A comparator monitors the SS pin voltage and
indicates the end of soft-start when SS pin voltage reaches 95%
of V
A T A
The simple RC soft-start allows the output to rise faster at the
SET
DC:DC C
. See Application Information section for further details.
S
SENSE
H E E T
= 2.5m
ONTROLLER
internal resistor. The capacitor voltage
Output voltage
V
Steady state voltage at high
current is approximately
V
Output current transient
step, I = 0 to 14A
(5A/Div)
OUT
SET
+ V
(50mV/Div)
OFFSET
- I
OUT
x R
LX1668
SENSE
7