ncp5050 ON Semiconductor, ncp5050 Datasheet - Page 9

• Manufacturer Part Number: ncp5050
• Description: 4.5 W Inductive Boost Led Driver For Flash And Lcd Backlight
• Manufacturer: ON Semiconductor

Operation
Mode PWM architecture which regulates the feedback
voltage at 250 mV under normal operating conditions. The
boost converter operates in two separate phases (See
Figure 19). The first one is T
by current from the battery to store up energy, followed by
T
external rectifier to the load. The capacitor C
store energy during the T
the load during the T
FB
FB REF
OFF
Iout
The NCP5050 DC-DC converter is based on a Current
SW
IL
step where the power is transmitted through the
COMP
RAMP
SHOT
ONE
Figure 19. Basic DC-DC Operation
-
+
UVLO REF
Cycle
Start
Ton
ON
588 ns
1.7 MHz
OSC
stage thus constantly powering the load.
M DUTY REF
OFF
-
+
ON
ENABLE
UVLO
COMP
time and to supply current to
when the inductor is charged
Toff
UVLO
MAX DUTY
CYCLE COMP
-
+
-
+
PWM
COMP
MAX D
DETAIL OPERATING DESCRIPTION
4.7uF X5R 6.3V
Figure 18. Functional Block Diagram
3
OUT
CTRL
250k
IPEAK MAX
is used to
C1
Ipeak
Ivalley
http://onsemi.com
PROTECTION
TIME
OUT
THERMAL
COMP
IPEAK
NCP5050
OVP
SET
RST
2.7 to 5.5 V
1
COMP
+
-
9
OVP
PVIN
trigger the PWM controller on each rising edge (SET signal)
which starts a cycle. During this phase the low side NMOS
switch is turned on thus increasing the current through the
inductor. The switch current is measured by the SENSE
CURRENT and added to the RAMP COMP signal. Then
PWM COMP compares the output of the adder and the
signal from ERROR AMP. When the comparator threshold
is exceeded, the NMOS switch is turned off until the rising
edge of the next clock cycle. In addition there are five
functions which can reset the flip-flop logic to switch off the
NMOS. The MAX DUTY CYCLE COMP monitors the
pulse width and if it exceeds 94% (nom) of the cycle time the
switch will be turned off. This limits the switch from being
on for more than one cycle. Thank to I
current through the inductor is monitored and compared
with the I
Selection). If the current exceeds this threshold the
controller is will turn off the NMOS switch for the remainder
of the cycle. This is a safety function to prevent any
excessive current that could overload the inductor and the
power stage. The three other safety circuits are, OVP,
UVLO, and THERMAL PROTECTION. Please refer to the
details in following sections.
built in integrator. The gain and the loop bandwidth are fixed
+
-
PCA
The internal oscillator provides a 1.7 MHz clock signal to
The loop stability is compensated by the ERROR AMP
2.7 mH
OVP REF
5
L1
R
2.8k
PCA
PEAK_MAX
Driver
CURRENT
SENSE
REF
PGND
threshold setup by R
10
1
R
SW
HCS
2.0
FB
6
8
10.0
R
LCS
MBR130LSF
750mW
250mW
D1
PCA
PEAK
2
7
4
1uF X5R 0805 25V
VS
FB
CM
(See Inductor
FLASH/TORCH
COMP, the
C2
C
out
D2
D5