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NCP5680
Auxiliary Operation
The power capability of the built-in charge pump makes
possible the use of a permanent load connected at the Vout
pin with a 100 mA continuous load current. Two situations
shall be considered:
a - the auxiliary load is ripple sensitive and fully active
when the power flash is triggered: using the extra PMOS
transistor is mandatory to isolate the auxiliary load from the
power LED. In this case, a minimum 100 m F/6.3 V
capacitor is required to maintain the Vout voltage constant
during the operation.
b - the auxiliary load is de-activated during a flash
sequence : using the PMOS transistor is not mandatory and
the auxiliary can be powered from the SuperCAP.
Thermal Consideration
Based on the data sheet published by the LED
manufacturers’, the forward drop of these LED can be 3 V
or less when the forward current is 100 mA and below.
Assuming that such a device is connected in the system, a
relative large amount of power will be dissipated into the
current mirrors built inside the NCP5680. Let assume that
Vf = 3.0 V @ If = 100 mA and consider the Vout was
programmed at 5.5 V. The voltage across the current mirror
will be:
Vom = Vout – Vf
Vom = 5.5 – 3 = 2.5 V
The power dissipated into the current mirror will be
Pom = Vom * If
Pom = 2.5 * 0.1 = 0.25 W
Assuming that the two LED are activated
simultaneously, the total power dissipated by the current
mirror will be 0.50 W. In addition, the charge pump is
active during this operation to maintain Vout at the
programmed value, and an extra 400 mW will be added to
the losses. On the other hand, it is unlikely possible to
increase the PCB area large enough to significantly reduce
the thermal resistance between the QFN package and the
ambient air: generally speaking, the Rthj ? a is in the range
of 40 ° C/W, assuming the Rthj ? c negligible in comparison
to the Rthc ? a. Taking all these parameters into account, the
die temperature can be calculated :
D Tj = SP x Rthj ? c
D Tj = (0.5 +0.4)*40 = 36 ° C
Assuming that the system operates under +85 ° C ambient
temperature, then the chip will be at 85+36 = 121 ° C, very
close to the maximum rating.
To avoid any long term fault with the system, it is
strongly recommended to reduce the Vout voltage to the
minimum value when the Torch/Video mode is activated.
In this case, the net voltage across the current mirror will
be limited to 4.5 – 3.0 = 1.5 V, the losses will decrease
accordingly, yielding a 113 ° C maximum junction
temperature under the same environment as depicted
before.
To control the Torch/Video mode, several registers must be preset before a flash takes place. A typical sequence is depicted here:
Select LED2
Select LED1
CONFIG0
B7
?
?
B6
?
?
B5
?
?
B4
?
?
B3
?
?
B2
?
?
B1
1/0
?
B0
?
1/0
Typical Sequence:
? send the I2C address
? select the CONFIG0 register
? activate LED1 and LED2
$7C
$01
$03
CONFIG1
Select the Charge Pump Mode of Operation:
0 3 charge pump de ? activated during a flash pulse
1 3 charge pump activated during a flash pulse
Select the Torch/Video mode
Control the charge pump: 0 3 charge pump de ? activated
Control the charge pump: 1 3 charge pump activated
Control the pre charge mode: 0 3 pre charge pump de ? activated
Control the pre charge mode: 1 3 pre charge pump activated
B7
?
?
?
?
B6
1
?
?
?
B5
?
0
?
?
B4
?
?
?
?
B3
?
?
?
?
B2
?
?
1
?
B1
?
?
?
1
B0
?
?
?
?
Indicator
The indicator mode is activated when the ENIND bit in
the CONFIG1 register is High. The pulse width and timer
registers are irrelevant, the time out is de ? activated. The
indicator function applies solely to LED 1. The Torch/
Video mode is bypassed if it was active in the previous
phase and resumes to the previous situation when ENIND
= Low.
The external NMOS are deactivated (Vgs1 = Vgs2 = 0)
and the ILED current is controlled by the built ? in current
mirror, the maximum current being 6.3 mA.
The indicator might be activated permanently, both the
power flash or the video/torch mode having an automatic
highest priority: the indicator current for LED1 is summed
with the main current coming from either the flash or the
video/torch circuits.
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