SC5014A View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC5014A
Semtech Corporation
'SC5014A' PDF : 27 Pages View PDF
SC5014A
Applications Information (continued)
Boost Converter Operation
The SC5014A includes a boost controller with program-
mable switching frequency. It applies a current-mode
control method with an integrated compensation loop as
shown in the diagram below. The clock (see block diagram
on page11) from the oscillator sets the latch and turns on
the external power MOSFET, which serves as the main
power switch. The current flowing through this switch is
sensed by the current sense resistor in series with the
switch. The sensed switch current is summed with the
slope-compensated ramp and fed into the modulating
input of the PWM comparator. When the modulating
ramp intersects the error amplifier output (COMP), the
latch is reset and the power MOSFET is turned off. The
sense resistor also sets the peak current limit of the power
MOSFET, IOCP using the following equation:
IOCP[A] = 0.4 / RCS [Ω]
VIN=4.5V to 27V
L1
Boost
O scillator
OSC
C o n tro l
Logic
NDRV
LS
CS
D1
Q1
PWM
COMP
S lo p e
Comp
+-+
COMP
+
EA
-
R4
DC
C5
IO
M in. Voltage
D e te c tio n
The current-mode control system contains two loops. For
the inner current loop, the error amplifier (EA) output
(COMP) controls the peak inductor current. In the outer
loop, the EA regulates the output voltage for driving the
LED strings.
Boost Converter Switching Frequency Selection
The resistor between FSET and GND sets the boost con-
verter switching frequency (200kHz to 2.2MHz) using the
following equation:
fSW [kHz] = 105/ RFSET [kΩ]
A higher switching frequency allows the use of low-profile
height inductors for space-constrained and cost-sensitive
applications.
Over-Voltage Protection (OVP)
The SC5014A features programmable output over-voltage
protection to prevent damage to the IC and output capaci-
tor in the event of a LED string open-circuit. The boost con-
verter output voltage is sensed at the OVP pin through the
resistor voltage divider. The OVP trip threshold (refer to
detailed application circuit on page 20, Figure 2) can be cal-
culated using the following equation:
OVP Trip Voltage [V] = 1.2 X (R11 + R12) / R12
When the OVP pin voltage exceeds 1.2V, the boost con-
verter turns off and the FLT pin is pulled low. When the
OVP pin voltage falls below the OVP threshold (falling),
the boost converter restarts and the FLT pin is released.
There is 10mV hysteresis between the OVP pin threshold
(falling) and the OVP pin threshold (rising). This results in
an output voltage hysteresis expressed as:
Output OVP Hysteresis [mV] = 10 X (R11 + R12) / R12
LED Current Sink
The SC5014A provides 2 current sinks and each can sink
up to 240mA current. It incorporates LED string short-
circuit protection (trip-level programmable; can be dis-
abled) and LED string open-circuit protection.
LED String Peak Current Programming
LED string peak current (at 100% dimming) can be set by
selecting resistor RISET, connected between ISET and GND.
The relationship between RISET resistance and single LED
string peak current is calculated using the following
equation:
ILED [mA] = 2 X (1036 X 1.23) / RISET [kΩ]
The string current can be programmed up to 240mA.
LED Connection
Two strings of LEDs can be connected to pin IO1 and pin
IO2. Pins IO1,2 showed stay connected.
Unused Strings
The SC5014A may be operated with less than 2 strings. In
this mode of operation, the unused IO pin should be con-
nected to ground. During start-up, these unused strings
are detected and disabled while other active strings work
normally, and FLT does not get pulled low.
13
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