LM3532TMX-40ANOPB National Semiconductor Corporation, LM3532TMX-40ANOPB Datasheet - Page 23

• Manufacturer Part Number: LM3532TMX-40ANOPB
• Description: High Efficiency White Led Driver With Programmable Ambient Light Sensing Capability And I2c-compatible Interface
• Manufacturer: National Semiconductor Corporation

INTERRUPT OUTPUT
INT is an open drain output that pulls low when the ALS is
enabled and when one of the ALS inputs transitions into a new
zone. At the same time, the ALS Zone Information register is
updated with the current ALS zone, and the software flag (bit
3 of the ALS Zone Information register) is written high. A
readback of the Zone Information Register will clear the soft-
ware interrupt flag and reset the INT output to the open drain
state. The active pulldown at INT is typically 125Ω.
Protection Features
OVERVOLTAGE PROTECTION
The LM3532’s boost converter provides open-load protection,
by monitoring the OVP pin. The OVP pin is designed to con-
nect as close as possible to the positive terminal of the output
capacitor. In the event of a disconnected load (LED current
string with feedback enabled), the output voltage will rise in
order to try and maintain the correct headroom across the
feedback enabled current sinks (see
climbs to the OVP threshold (V
turned off, and switching will stop until VOUT falls below the
OVP hysteresis (V
crossed the LM3532’s boost converter begins switching
again. In open load conditions this would result in a pulsed
on/off operation.
CURRENT LIMIT
The LM3532’s peak current limit in the NFET is set at typically
1A (880 mA min.). During the positive portion of the switching
cycle, if the NFET's current rises up to the current limit thresh-
old, the NFET turns off for the rest of the switching cycle. At
the start of the next switching cycle the NFET turns on again.
For loads that cause the LM3532 to hit current limit each
switching cycle, the output power can become clamped since
the headroom across the feedback enabled current sinks is
no longer being regulated when the device is in current limit.
See
how peak current affects the LM3532's maximum output pow-
er.
MAXIMUM OUTPUT POWER
The LM3532's maximum output power is governed by two
factors: the peak current limit (I
maximum output voltage (V
cation causes either of these limits to be reached it is possible
that the proper current regulation and matching between LED
current strings will not be met.
Peak Current Limited
In the case of a peak current limited situation, when the peak
of the inductor current hits the LM3532's current limit the
NFET switch turns off for the remainder of the switching pe-
MAXIMUM OUTPUT POWER
OVP
– 1V). Once the OVP hysteresis is
OVP
= 40V min.). When the appli-
CL
OVP
= 880 mA min.), and the
) the boost converter is
below for guidelines on
Table
13). Once VOUT
23
riod. If this happens, each switching cycle the LM3532 begins
to regulate the peak of the inductor current instead of the
headroom across the current sinks. This can result in the
dropout of the feedback-enabled current sinks and the current
dropping below its programmed level.
The peak current in a boost converter is dependent on the
value of the inductor, total LED current (IOUT), the output
voltage (VOUT) (which is the highest voltage LED string +
0.4V regulated headroom voltage), the input voltage VIN, and
the efficiency (Output Power/Input Power). Additionally, the
peak current is different depending on whether the inductor
current is continuous during the entire switching period (CCM)
or discontinuous (DCM) where it goes to 0 before the switch-
ing period ends.
For Continuous Conduction Mode the peak inductor current
is given by:
For Discontinuous Conduction Mode the peak inductor cur-
rent is given by:
To determine which mode the circuit is operating in (CCM or
DCM) it is necessary to perform a calculation to test whether
the inductor current ripple is less than the anticipated input
current (IIN). If ΔIL is < then IIN then the device will be oper-
ating in CCM. If ΔIL is > IIN then the device is operating in
DCM.
Typically at currents high enough to reach the LM3532's peak
current limit, the device will be operating in CCM.
The following figures show the output current and voltage de-
rating for a 10 µH and a 22 µH inductor. These plots take
equations (1) and (2) from above and plot VOUT and IOUT
with varying VIN, a constant peak current of 880 mA (ICL min),
and a constant efficiency of 85%. Using these curves can give
a good design guideline on selecting the correct inductor for
a given output power requirement. A 10 µH will typically be a
smaller device with lower on resistance, but the peak currents
will be higher. A 22 µH provides for lower peak currents, but
to match the DC resistance of a 10 µH requires a larger sized
device.
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