LM34919TL [NSC]
40V, 600 mA Step Down COT Switching Regulator; 40V ， 600毫安降压COT开关稳压器
| 型号： | LM34919TL |
| 厂家： | 
National Semiconductor |
| 描述： | 40V, 600 mA Step Down COT Switching Regulator |
| 文件： | 总18页 (文件大小：365K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
November 1, 2007
LM34919
40V, 600 mA Step Down COT Switching Regulator
No loop compensation required
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General Description
Ultra-Fast transient response
The LM34919 Step Down Switching Regulator features all of
Operating frequency remains constant with load current
and input voltage
the functions needed to implement a low cost, efficient, buck
bias regulator capable of supplying 0.6A to the load. This buck
regulator contains an N-Channel Buck Switch, and is avail-
able in a micro SMD package. The constant on-time feedback
regulation scheme requires no loop compensation, results in
fast load transient response, and simplifies circuit implemen-
tation. The operating frequency remains constant with line
and load variations due to the inverse relationship between
the input voltage and the on-time. The valley current limit re-
sults in a smooth transition from constant voltage to constant
current mode when current limit is detected, reducing the fre-
quency and output voltage, without the use of foldback. Ad-
ditional features include: VCC under-voltage lockout, thermal
shutdown, gate drive under-voltage lockout, and maximum
duty cycle limiter.
Maximum switching frequency: 1.6 MHz
Maximum Duty Cycle Limited During Start-Up
Adjustable output voltage
Valley Current Limit At 0.64A
Precision internal reference
Low bias current
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Highly efficient operation
Thermal shutdown
Typical Applications
High Efficiency Point-Of-Load (POL) Regulator
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Non-Isolated Telecommunication Buck Regulator
Features
Secondary High Voltage Post Regulator
Integrated N-Channel buck switch
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Package
Integrated start-up regulator
Input Voltage Range: 8V to 40V
micro SMD
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Basic Step Down Regulator
30004431
© 2007 National Semiconductor Corporation
300044
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Connection Diagrams
30004433
Top View
30004402
Bump Side
Ordering Information
Order Number
Package Type
NSC Package Drawing Junction Temperature Range
Supplied As
LM34919TL
10-Bump micro SMD
TLP10A1A
−40°C to + 125°C
250 Units on Tape and
Reel
LM34919TLX
10-Bump micro SMD
TLP10A1A
−40°C to + 125°C
3000 Units on Tape and
Reel
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Pin Descriptions
Pin Number
Name
Description
Application Information
A1
RON/SD
On-time control and shutdown
An external resistor from VIN to this pin sets the buck switch
on-time. Grounding this pin shuts down the regulator.
A2
A3
B1
B3
C1
RTN
FB
Circuit Ground
Ground for all internal circuitry other than the current limit
detection.
Feedback input from the regulated
output
Internally connected to the regulation and over-voltage
comparators. The regulation level is 2.5V.
SGND
SS
Sense Ground
Re-circulating current flows into this pin to the current sense
resistor.
Softstart
An internal current source charges an external capacitor to
2.5V, providing the softstart function.
ISEN
Current sense
The re-circulating current flows through the internal sense
resistor, and out of this pin to the free-wheeling diode.
Current limit is nominally set at 0.64A.
C3
VCC
Output from the startup regulator
Nominally regulates at 7.0V. An external voltage (7V-14V)
can be applied to this pin to reduce internal dissipation. An
internal diode connects VCC to VIN.
D1
D2
VIN
SW
Input supply voltage
Switching Node
Nominal input range is 8.0V to 40V.
Internally connected to the buck switch source. Connect to
the inductor, free-wheeling diode, and bootstrap capacitor.
D3
BST
Boost pin for bootstrap capacitor
Connect a 0.022 µF capacitor from SW to this pin. The
capacitor is charged from VCC via an internal diode during
each off-time.
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BST to SW
VCC to RTN
SGND to RTN
SS to RTN
All Other Inputs to RTN
Storage Temperature Range
JunctionTemperature
14V
14V
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
-0.3V to +0.3V
-0.3V to 4V
-0.3 to 7V
-65°C to +150°C
150°C
VIN to RTN
44V
BST to RTN
52V
SW to RTN (Steady State)
ESD Rating (Note 2)
Human Body Model
BST to VCC
-1.5V
Operating Ratings (Note 1)
VIN
2kV
44V
44V
8.0V to 40V
−40°C to + 125°C
Junction Temperature
VIN to SW
Electrical Characteristics Specifications with standard type are for TJ = 25°C only; limits in boldface type apply
over the full Operating Junction Temperature (TJ) range. Minimum and Maximum limits are guaranteed through test, design, or
statistical correlation. Typical values represent the most likely parametric norm at TJ = 25°C, and are provided for reference
purposes only. Unless otherwise stated the following conditions apply: VIN = 12V, RON = 200kΩ. See (Note 5).
Symbol
Parameter
Conditions
Min
Typ
Max
Units
Start-Up Regulator, VCC
VCCReg
VCC regulated output
6.6
7
7.4
V
V
VIN-VCC dropout voltage
ICC = 0 mA,
1.2
VCC = UVLOVCC + 250 mV
VCC output impedance
175
Ω
mA
V
0 mA ≤ ICC ≤ 5 mA, VIN = 8V
VCC = 0V
VCC current limit (Note 3)
9.5
5.7
150
3
UVLOVCC VCC under-voltage lockout threshold VCC increasing
UVLOVCC hysteresis
UVLOVCC filter delay
IIN operating current
IIN shutdown current
VCC decreasing
mV
µs
100 mV overdrive
IQ
Non-switching, FB = 3V, SW = Open
RON/SD = 0V, SW = Open
0.5
75
0.8
mA
µA
ISD
150
Switch Characteristics
Rds(on)
UVLOGD
Buck Switch Rds(on)
ITEST = 200 mA
0.5
4.4
1.0
5.2
Ω
V
Gate Drive UVLO
VBST - VSW Increasing
3.0
UVLOGD hysteresis
480
mV
Softstart Pin
VSS
Pull-up voltage
2.5
V
Internal current source
VSS = 1V
10.5
µA
Current Limit
ILIM
Threshold
Current out of ISEN
0.52
0.64
140
150
0.76
A
Resistance from ISEN to SGND
Response time
mΩ
ns
On Timer
tON - 1
On-time
On-time
2.1
2.77
700
3.5
1.2
µs
ns
VIN = 10V, RON = 200 kΩ
tON - 2
VIN = 40V, RON = 200 kΩ
Voltage at RON/SD rising
Voltage at RON/SD
Shutdown threshold
Threshold hysteresis
0.45
0.8
25
V
mV
Off Timer
tOFF
Minimum Off-time
155
ns
Regulation and Over-Voltage Comparators (FB Pin)
VREF
FB regulation threshold
FB over-voltage threshold
FB bias current
SS pin = steady state
2.440
2.5
2.9
1
2.550
V
V
FB = 3V
nA
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Symbol
Parameter
Conditions
Min
Typ
Max
Units
Thermal Shutdown
TSD
Thermal shutdown temperature
Thermal shutdown hysteresis
175
20
°C
°C
Thermal Resistance
Junction to Ambient
0 LFPM Air Flow
61
°C/W
θJA
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the
device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin.
Note 3: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading
Note 4: For detailed information on soldering micro SMD package, refer to the Application Note AN-1112.
Note 5: Typical specifications represent the most likely parametric norm at 25°C operation.
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Typical Performance Characteristics
VCC vs VIN
VCC vs ICC
30004405
30004404
ICC vs Externally Applied VCC
ON-TIME vs VIN and RON
30004435
30004436
Voltage at the RON/SD Pin
Shutdown and Operating Current into VIN
30004437
30004438
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Typical Application Circuit and Block Diagram
30004401
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30004434
FIGURE 1. Start Up Sequence
Functional Description
Control Circuit Overview
The LM34919 Step Down Switching Regulator features all the
functions needed to implement a low cost, efficient buck bias
power converter capable of supplying at least 0.6A to the load.
This high voltage regulator contains an N-Channel buck
switch, is easy to implement, and is available in micro SMD
package. The regulator’s operation is based on a constant on-
time control scheme, where the on-time is determined by
VIN. This feature allows the operating frequency to remain
relatively constant with load and input voltage variations. The
feedback control requires no loop compensation resulting in
very fast load transient response. The valley current limit de-
tection circuit, internally set at 0.64A, holds the buck switch
off until the high current level subsides. This scheme protects
against excessively high current if the output is short-circuited
when VIN is high.
The LM34919 buck DC-DC regulator employs a control
scheme based on a comparator and a one-shot on-timer, with
the output voltage feedback (FB) compared to an internal ref-
erence (2.5V). If the FB voltage is below the reference the
buck switch is turned on for a time period determined by the
input voltage and a programming resistor (RON). Following the
on-time the switch remains off until the FB voltage falls below
the reference but not less than the minimum off-time. The
buck switch then turns on for another on-time period. Typi-
cally, during start-up, or when the load current increases
suddenly, the off-times are at the minimum. Once regulation
is established, the off-times are longer.
When in regulation, the LM34919 operates in continuous con-
duction mode at heavy load currents and discontinuous con-
duction mode at light load currents. In continuous conduction
mode current always flows through the inductor, never reach-
ing zero during the off-time. In this mode the operating fre-
quency remains relatively constant with load and line
variations. The minimum load current for continuous conduc-
The LM34919 can be applied in numerous applications to ef-
ficiently regulate down higher voltages. Additional features
include: Thermal shutdown, VCC under-voltage lockout, gate
drive under-voltage lockout, and maximum duty cycle limiter.
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tion mode is one-half the inductor’s ripple current amplitude.
The operating frequency is approximately:
VOUT = 2.5 x (R1 + R2) / R2
Output voltage regulation is based on ripple voltage at the
feedback input,normally obtained from the output voltage rip-
ple through the feedback resistors. The LM34919 requires a
minimum of 25 mV of ripple voltage at the FB pin. In cases
where the capacitor’s ESR is insufficient additional series re-
sistance may be required (R3).
(1)
The buck switch duty cycle is approximately equal to:
Start-Up Regulator, VCC
The start-up regulator is integral to the LM34919. The input
pin (VIN) can be connected directly to line voltage up to 40V,
with transient capability to 44V. The VCC output regulates at
7.0V, and is current limited at 9.5 mA. Upon power up, the
regulator sources current into the external capacitor at VCC
(C3). When the voltage on the VCC pin reaches the under-
voltage lockout threshold of 5.7V, the buck switch is enabled
and the Softstart pin is released to allow the Softstart capac-
itor (C6) to charge up.
(2)
In discontinuous conduction mode current through the induc-
tor ramps up from zero to a peak during the on-time, then
ramps back to zero before the end of the off-time. The next
on-time period starts when the voltage at FB falls below the
reference - until then the inductor current remains zero, and
the load current is supplied by the output capacitor. In this
mode the operating frequency is lower than in continuous
conduction mode, and varies with load current. Conversion
efficiency is maintained at light loads since the switching loss-
es decrease with the reduction in load and frequency. The
approximate discontinuous operating frequency can be cal-
culated as follows:
The minimum input voltage is determined by the regulator’s
dropout voltage, the VCC UVLO falling threshold (≊5.55V),
and the frequency. When VCC falls below the falling threshold
the VCC UVLO activates to shut off the output. If VCC is exter-
nally loaded, the minimum input voltage increases.
To reduce power dissipation in the start-up regulator, an aux-
iliary voltage can be diode connected to the VCC pin. Setting
the auxiliary voltage to between 7V and 14V shuts off the in-
ternal regulator, reducing internal power dissipation. The sum
of the auxiliary voltage and the input voltage (VCC + VIN) can-
not exceed 52V. Internally, a diode connects VCC to VIN. See
Figure 2.
(3)
where RL = the load resistance.
The output voltage is set by two external resistors (R1, R2).
The regulated output voltage is calculated as follows:
30004411
FIGURE 2. Self Biased Configuration
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Regulation Comparator
Current Limit
The feedback voltage at FB is compared to the voltage at the
Softstart pin (2.5V). In normal operation (the output voltage is
regulated), an on-time period is initiated when the voltage at
FB falls below 2.5V. The buck switch stays on for the pro-
grammed on-time, causing the FB voltage to rise above 2.5V.
After the on-time period, the buck switch stays off until the FB
voltage falls below 2.5V. Input bias current at the FB pin is
less than 100 nA over temperature.
Current limit detection occurs during the off-time by monitor-
ing the recirculating current through the free-wheeling diode
(D1). Referring to the Block Diagram, when the buck switch
is turned off the inductor current flows through the load, into
SGND, through the sense resistor, out of ISEN and through
D1. If that current exceeds 0.64A the current limit comparator
output switches to delay the start of the next on-time period.
The next on-time starts when the current out of ISEN is below
0.64A and the voltage at FB is below 2.5V. If the overload
condition persists causing the inductor current to exceed
0.64A during each on-time, that is detected at the beginning
of each off-time. The operating frequency is lower due to
longer-than-normal off-times.
Over-Voltage Comparator
The voltage at FB is compared to an internal 2.9V reference.
If the voltage at FB rises above 2.9V the on-time pulse is im-
mediately terminated. This condition can occur if the input
voltage or the output load changes suddenly, or if the inductor
(L1) saturates. The buck switch remains off until the voltage
at FB falls below 2.5V.
Figure 4 illustrates the inductor current waveform. During nor-
mal operation the load current is Io, the average of the ripple
waveform. When the load resistance decreases the current
ratchets up until the lower peak reaches 0.64A. During the
Current Limited portion of Figure 4, the current ramps down
to 0.64A during each off-time, initiating the next on-time (as-
suming the voltage at FB is <2.5V). During each on-time the
current ramps up an amount equal to:
ON-Time Timer, and Shutdown
The on-time is determined by the RON resistor and the input
voltage (VIN), and is calculated from:
ΔI = (VIN - VOUT) x tON / L1
During this time the LM34919 is in a constant current mode,
with an average load current (IOCL) equal to 0.64A + ΔI/2.
(4)
Generally, in applications where the switching frequency is
higher than ≊300 kHz and uses a small value inductor, the
higher dl/dt of the inductor's ripple current results in an effec-
tively lower valley current limit threshold due to the response
time of the current limit detection circuit. However, since the
small value inductor results in a relatively high ripple current
amplitude (ΔI in Figure 4), the load current (IOCL) at current
limit is typically in excess of 640 mA.
The inverse relationship with VIN results in a nearly constant
frequency as VIN is varied. To set a specific continuous con-
duction mode switching frequency (FS), the RON resistor is
determined from the following:
(5)
In high frequency applicatons the minimum value for tON is
limited by the maximum duty cycle required for regulation and
the minimum off-time of (155 ns, ±15%). The minimum off-
time limits the maximum duty cycle achievable with a low
voltage at VIN. At high values of VIN, the minimum on-time is
limited to ≊ 120 ns.
The LM34919 can be remotely shut down by taking the RON/
SD pin below 0.8V. See Figure 3. In this mode the SS pin is
internally grounded, the on-timer is disabled, and bias cur-
rents are reduced. Releasing the RON/SD pin allows normal
operation to resume. The voltage at the RON/SD pin is be-
tween 1.4V and 4.0V, depending on VIN and the RON resistor.
30004413
FIGURE 3. Shutdown Implementation
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30004414
FIGURE 4. Inductor Current - Current Limit Operation
N - Channel Buck Switch and Driver
Applications Information
The LM34919 integrates an N-Channel buck switch and as-
sociated floating high voltage gate driver. The peak current
allowed through the buck switch is 1.5A, and the maximum
allowed average current is 1A. The gate driver circuit works
in conjunction with an external bootstrap capacitor and an in-
ternal high voltage diode. A 0.022 µF capacitor (C4) connect-
ed between BST and SW provides the voltage to the driver
during the on-time. During each off-time, the SW pin is at ap-
proximately -1V, and C4 charges from VCC through the inter-
nal diode. The minimum off-time forced by the LM34919
ensures a minimum time each cycle to recharge the bootstrap
capacitor.
EXTERNAL COMPONENTS
The procedure for calculating the external components is il-
lustrated with the following design example. Referring to the
Block Diagram, the circuit is to be configured for the following
specifications:
- VOUT = 5V
- VIN = 8V to 40V
- Minimum load current = 200 mA
- Maximum load current = 600 mA
- Switching Frequency = 800 kHz
- Soft-start time = 5 ms
R1 and R2: These resistors set the output voltage. The ratio
of the feedback resistors is calculated from:
Softstart
The softstart feature allows the converter to gradually reach
a steady state operating point, thereby reducing start-up
stresses and current surges. Upon turn-on, after VCC reaches
the under-voltage threshold, an internal 10.5 µA current
source charges up the external capacitor at the SS pin to
2.5V. The ramping voltage at SS (and the non-inverting input
of the regulation comparator) ramps up the output voltage in
a controlled manner.
R1/R2 = (VOUT/2.5V) - 1
For this example, R1/R2 = 1. R1 and R2 should be chosen
from standard value resistors in the range of 1.0 kΩ - 10 kΩ
which satisfy the above ratio. For this example, 2.49kΩ is
chosen for R1 and R2.
RON: This resistor sets the on-time, and (by default) the
switching frequency. The switching frequncy must be less
than 1.6 MHz to ensure the minimum forced off-time does not
interfere with the circuit's proper operation. The RON resistor
is calculated from the following equation, using the minimum
input voltage.
An internal switch grounds the SS pin if VCC is below the un-
der-voltage lockout threshold, or if the RON/SD pin is ground-
ed.
Thermal Shutdown
The LM34919 should be operated so the junction temperature
does not exceed 125°C. If the junction temperature increases,
an internal Thermal Shutdown circuit, which activates (typi-
cally) at 175°C, takes the controller to a low power reset state
by disabling the buck switch. This feature helps prevent catas-
trophic failures from accidental device overheating. When the
junction temperature reduces below 155°C (typical hysteresis
= 20°C) normal operation resumes.
Check that this value resistor does not set an on-time less
than 120 ns at maximum VIN.
A standard value 43.2 kΩ resistor is used, resulting in a nom-
inal frequency of 806 kHz. The minimum on-time is ≊231 ns
at Vin = 40V, and the maximum on-time is ≊875 ns at Vin =
8V. Alternately, RON can be determined using Equation 4 if a
specific on-time is required.
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L1: The main parameter affected by the inductor is the in-
ductor current ripple amplitude (IOR). The minimum load cur-
rent is used to determine the maximum allowable ripple in
order to maintain continuous conduction mode, where the
lower peak does not reach 0 mA. This is not a requirement of
the LM34919, but serves as a guideline for selecting L1. For
this case the maximum ripple current is:
inductor’s ripple current, ramps up to the upper peak, then
drops to zero at turn-off. The average current during the on-
time is the load current. For a worst case calculation, C1 must
supply this average load current during the maximum on-time,
without letting the voltage at VIN drop below ≊7.5V. The min-
imum value for C1 is calculated from:
IOR(MAX) = 2 x IOUT(min) = 400 mA
(6)
If the minimum load current is zero, use 20% of IOUT(max) for
IOUT(min) in equation 6. The ripple calculated in Equation 6 is
then used in the following equation:
where tON is the maximum on-time, and ΔV is the allowable
ripple voltage (0.5V at VIN = 8V). C5’s purpose is to minimize
transients and ringing due to long lead inductance leading to
the VIN pin. A low ESR, 0.1 µF ceramic chip capacitor must
be located close to the VIN and RTN pins.
(7)
A standard value 15 µH inductor is selected. The maximum
ripple amplitude, which occurs at maximum VIN, calculates to
362 mA p-p, and the peak current is 781 mA at maximum load
current. Ensure the selected inductor is rated for this peak
current.
C3: The capacitor at the VCC pin provides noise filtering and
stability for the Vcc regulator. C3 should be no smaller than
0.1 µF, and should be a good quality, low ESR, ceramic ca-
pacitor. C3’s value, and the VCC current limit, determine a
portion of the turn-on-time (t1 in Figure 1).
C2 and R3: Since the LM34919 requires a minimum of 25
mVp-p ripple at the FB pin for proper operation, the required
ripple at VOUT is increased by R1 and R2. This necessary rip-
ple is created by the inductor ripple current flowing through
R3, and to a lesser extent by C2 and its ESR. The minimum
inductor ripple current is calculated using equation 7, rear-
ranged to solve for IOR at minimum VIN.
C4: The recommended value for C4 is 0.022 µF. A high quality
ceramic capacitor with low ESR is recommended as C4 sup-
plies a surge current to charge the buck switch gate at each
turn-on. A low ESR also helps ensure a complete recharge
during each off-time.
C6: The capacitor at the SS pin determines the softstart time,
i.e. the time for the output voltage, to reach its final value (t2
in Figure 1). The capacitor value is determined from the fol-
lowing:
The minimum value for R3 is equal to:
D1: A Schottky diode is recommended. Ultra-fast recovery
diodes are not recommended as the high speed transitions at
the SW pin may inadvertently affect the IC’s operation through
external or internal EMI. The diode should be rated for the
maximum input voltage, the maximum load current, and the
peak current which occurs when the current limit and maxi-
mum ripple current are reached simultaneously. The diode’s
average power dissipation is calculated from:
A standard value 0.39Ω resistor is used for R3 to allow for
tolerances. C2 should generally be no smaller than 3.3 µF,
although that is dependent on the frequency and the desired
output characteristics. C2 should be a low ESR good quality
ceramic capacitor. Experimentation is usually necessary to
determine the minimum value for C2, as the nature of the load
may require a larger value. A load which creates significant
transients requires a larger value for C2 than a non-varying
load.
PD1 = VF x IOUT x (1-D)
where VF is the diode’s forward voltage drop, and D is the on-
time duty cycle.
FINAL CIRCUIT
C1 and C5: C1’s purpose is to supply most of the switch cur-
rent during the on-time, and limit the voltage ripple at VIN, on
the assumption that the voltage source feeding VIN has an
output impedance greater than zero.
The final circuit is shown in Figure 5, and its performance is
shown in Figure 6 and Figure 7. Current limit measured ap-
proximately 650 mA at 8V, and 740 mA at 40V.
At maximum load current, when the buck switch turns on, the
current into VIN suddenly increases to the lower peak of the
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30004421
FIGURE 5. Example Circuit
30004440
FIGURE 6. Efficiency vs. Load Current and VIN (Circuit of Figure 5)
30004423
FIGURE 7. Frequency vs. VIN (Circuit of Figure 5)
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LOW OUTPUT RIPPLE CONFIGURATIONS
For applications where lower ripple at VOUT is required, the
following options can be used to reduce or nearly eliminate
the ripple.
a) Reduced ripple configuration: In Figure 8, Cff is added
across R1 to AC-couple the ripple at VOUT directly to the FB
pin. This allows the ripple at VOUT to be reduced to a minimum
of 25 mVp-p by reducing R3, since the ripple at VOUT is not
attenuated by the feedback resistors. The minimum value for
Cff is determined from:
30004427
FIGURE 9. Minimum Output Ripple Using Ripple Injection
c) Alternate minimum ripple configuration: The circuit in
Figure 10 is the same as that in Figure 5, except the output
voltage is taken from the junction of R3 and C2. The ripple at
VOUT is determined by the inductor’s ripple current and C2’s
characteristics. However, R3 slightly degrades the load reg-
ulation. This circuit may be suitable if the load current is fairly
constant.
where tON(max) is the maximum on-time, which occurs at VIN
(min). The next larger standard value capacitor should be used
for Cff. R1 and R2 should each be towards the upper end of
the 2 kΩ to 10 kΩ range.
30004428
30004425
FIGURE 10. Alternate Minimum Output Ripple
Configuration
FIGURE 8. Reduced Ripple Configuration
b) Minimum ripple configuration: The circuit of Figure 9
provides minimum ripple at VOUT, determined primarily by
C2’s characteristics and the inductor’s ripple current since R3
is removed. RA and CA are chosen to generate a sawtooth
waveform at their junction, and that voltage is AC-coupled to
the FB pin via CB. To determine the values for RA, CA and
CB, use the following procedure:
Minimum Load Current
The LM34919 requires a minimum load current of 1 mA. If the
load current falls below that level, the bootstrap capacitor (C4)
may discharge during the long off-time, and the circuit will ei-
ther shutdown, or cycle on and off at a low frequency. If the
load current is expected to drop below 1 mA in the application,
R1 and R2 should be chosen low enough in value so they
provide the minimum required current at nominal VOUT
.
Calculate VA = VOUT - (VSW x (1 - (VOUT/VIN(min))))
where VSW is the absolute value of the voltage at the SW pin
during the off-time (typically 1V). VA is the DC voltage at the
RA/CA junction, and is used in the next equation.
PC BOARD LAYOUT
Refer to application note AN-1112 for PC board guidelines for
the Micro SMD package.
The LM34919 regulation, over-voltage, and current limit com-
parators are very fast, and respond to short duration noise
pulses. Layout considerations are therefore critical for opti-
mum performance. The layout must be as neat and compact
as possible, and all of the components must be as close as
possible to their associated pins. The two major current loops
have currents which switch very fast, and so the loops should
be as small as possible to minimize conducted and radiated
EMI. The first loop is that formed by C1, through the VIN to
SW pins, L1, C2, and back to C1.The second current loop is
formed by D1, L1, C2 and the SGND and ISEN pins.
where tON is the maximum on-time (at minimum input volt-
age), and ΔV is the desired ripple amplitude at the RA/CA
junction, typically 100 mV. RA and CA are then chosen from
standard value components to satisfy the above product. Typ-
ically CA is 3000 pF to 5000 pF, and RA is 10 kΩ to 300 kΩ.
CB is then chosen large compared to CA, typically 0.1 µF. R1
and R2 should each be towards the upper end of the 2 kΩ to
10 kΩ range.
The power dissipation within the LM34919 can be approxi-
mated by determining the total conversion loss (PIN - POUT),
and then subtracting the power losses in the free-wheeling
diode and the inductor. The power loss in the diode is ap-
proximately:
PD1 = Iout x VF x (1-D)
where Iout is the load current, VF is the diode’s forward volt-
age drop, and D is the on-time duty cycle. The power loss in
the inductor is approximately:
www.national.com
14
PL1 = Iout2 x RL x 1.1
ditionally the use of wide PC board traces, where possible,
can help conduct heat away from the IC. Judicious positioning
of the PC board within the end product, along with the use of
any available air flow (forced or natural convection) can help
reduce the junction temperatures.
where RL is the inductor’s DC resistance, and the 1.1 factor
is an approximation for the AC losses. If it is expected that the
internal dissipation of the LM34919 will produce excessive
junction temperatures during normal operation, good use of
the PC board’s ground plane can help to dissipate heat. Ad-
15
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Physical Dimensions inches (millimeters) unless otherwise noted
Note: X1 = 1.514 mm, ±0.030 mm
X2 = 1.970 mm, ±0.030 mm
X3 = 0.60 mm, ±0.075 mm
10 Bump micro SMD Package
NS Package Number TLP10A1A
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16
Notes
17
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Notes
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