SC5014
Applications Information (continued)
Boost Converter Operation
The SC5014 includes a boost controller with programma-
ble 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-com-
pensated 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=4.5 to 27V
L1
Boost
Oscillator
OSC
Control
Logic
PWM
COMP
Slope
Comp
+-+
COMP
+
EA
-
NDRV
LS
CS
D1
Q1
RCS
DC
C5
IO
Min. Voltage
Detection
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 SC5014 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 23, 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 SC5014 provides 4 current sinks and each can sink up
to 120mA current. It incorporates LED string short-circuit
protection (trip-level programmable; can be disabled)
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 120mA.
Unused Strings
The SC5014 may be operated with less than 4 strings. In
this mode of operation, all unused IO pins 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.
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