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PDF LM3409QHV Data sheet ( Hoja de datos )
| Número de pieza | LM3409QHV | |
| Descripción | PFET Buck Controller | |
| Fabricantes | National Semiconductor | |
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LM3409 / LM3409HV
LM3409Q / LM3409QHV
April 12, 2010
PFET Buck Controller for High Power LED Drives
General Description
The LM3409/09HV are P-channel MosFET (PFET) con-
trollers for step-down (buck) current regulators. They offer
wide input voltage range, high-side differential current sense
with low adjustable threshold voltage, fast output enable/dis-
able function and a thermally enhanced eMSOP-10 package.
These features combine to make the LM3409/09HV ideal for
use as constant current sources for driving LEDs where for-
ward currents up to 5A are easily achievable. The
LM3409/09Q/09HV/09QHV uses Constant Off-Time (COFT)
control to regulate an accurate constant current without the
need for external control loop compensation. Analog and
PWM dimming are easy to implement and result in a highly
linear dimming range with excellent achievable contrast ra-
tios. Programmable UVLO, low-power shutdown, and thermal
shutdown complete the feature set.
Features
■ LM3409Q/LM3409QHV is an Automotive Grade product
that is AEC-Q100 grade 1 qualified
■ 2Ω, 1A Peak MosFET Gate Drive
■ VIN Range: 6V to 42V (LM3409/LM3409Q)
■ VIN Range: 6V to 75V (LM3409HV/LM3409QHV)
■ Differential, High-side Current Sense
■ Cycle-by-Cycle Current Limit
■ No Control Loop Compensation Required
■ 10,000:1 PWM Dimming Range
■ 250:1 Analog Dimming Range
■ Supports All-Ceramic Output Capacitors and Capacitor-
less Outputs
■ Low Power Shutdown
■ Thermal Shutdown Protection
■ Thermally Enhanced eMSOP-10 Package
Applications
■ LED Driver
■ Constant Current Source
■ Automotive Lighting
■ General Illumination
■ Industrial Lighting
Typical Application
© 2010 National Semiconductor Corporation 300856
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Typical Performance Characteristics
TA = +25°C, VIN = 24V, and characteristics are identical for LM3409 and LM3409HV unless otherwise specified.
VCST vs. Junction Temperature
VCC vs. Junction Temperature
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VADJ vs. Junction Temperature
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IADJ vs. Junction Temperature
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VOFT vs. Junction Temperature
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tON-MIN vs. Junction Temperature
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AVERAGE LED CURRENT
For a buck converter, the average LED current is simply the
average inductor current.
INDUCTOR CURRENT RIPPLE
Because the LM3409/09HV swaps the polarity of the differ-
ential current sense comparator every cycle, a minimum in-
ductor current ripple (ΔiL-PP) is necessary to maintain
accurate ILED regulation. Referring to Figure 6, the first tON is
terminated at the higher of the two polarity-swapped thresh-
olds (corresponding to IL-MAXH). During the following tOFF, iL
decreases until the second tON begins. If tOFF is too short, then
as the second tON begins, iL will still be above the lower peak
current threshold (corresponding to IL-MAXL) and a minimum
tON pulse will follow. This will result in degraded ILED regula-
tion. The minimum inductor current ripple (ΔiL-PP-MIN) should
adhere to the following equation in order to ensure accurate
ILED regulation:
30085611
FIGURE 5. Sense Voltage vSNS(t)
Using the COFT architecture, the peak transistor current (IT-
MAX) is sensed as shown in Figure 5, which is equal to the
peak inductor current (IL-MAX) given by the following equation:
Because IL-MAX is set using peak current control and ΔiL-PP is
set using the controlled off-timer, IL and correspondingly
ILED can be calculated as follows:
The threshold voltage VCST seen by the high-side sense com-
parator is affected by the comparator’s input offset voltage,
which causes an error in the calculation of IL-MAX and ulti-
mately ILED. To mitigate this problem, the polarity of the com-
parator inputs is swapped every cycle, which causes the
actual IL-MAX to alternate between two peak values (IL-MAXH
and IL-MAXL), equidistant from the theoretical IL-MAX as shown
in Figure 6. ILED remains accurate through this averaging.
SWITCHING FREQUENCY
The switching frequency is dependent upon the actual oper-
ating point (VIN and VO). VO will remain relatively constant for
a given application, therefore the switching frequency will vary
with VIN (frequency increases as VIN increases). The target
switching frequency (fSW) at the nominal operating point is
selected based on the tradeoffs between efficiency (better at
low frequency) and solution size/cost (smaller at high fre-
quency). The off-time of the LM3409/09HV can be pro-
grammed for switching frequencies up to 5 MHz (theoretical
limit imposed by minimum tON). In practice, switching fre-
quencies higher than 1MHz may be difficult to obtain due to
gate drive limitations, high input voltage, and thermal consid-
erations.
At CCM operating points, fSW is defined as:
At DCM operating points, fSW is defined as:
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FIGURE 6. Inductor Current iL(t) Showing IL-MAX Offset
In the CCM equation, it is apparent that the efficiency (η) fac-
tors into the switching frequency calculation. Efficiency is hard
to estimate and, since switching frequency varies with input
voltage, accuracy in setting the nominal switching frequency
is not critical. Therefore, a general rule of thumb for the
LM3409/09HV is to assume an efficiency between 85% and
100%. When approximating efficiency to target a nominal
switching frequency, the following condition must be met:
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| PDF Descargar | [ Datasheet LM3409QHV.PDF ] | |
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