LM34919BQTLX [NSC]
Ultra Small 40V, 600 mA Constant On-Time Buck Switching Regulator; 超小型40V ， 600毫安恒定导通时间降压型开关稳压器
| 型号： | LM34919BQTLX |
| 厂家： | 
National Semiconductor |
| 描述： | Ultra Small 40V, 600 mA Constant On-Time Buck Switching Regulator |
| 文件： | 总20页 (文件大小：483K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
May 26, 2010
LM34919B
Ultra Small 40V, 600 mA Constant On-Time Buck Switching
Regulator
General Description
Features
The LM34919B Step Down Switching Regulator features all
of 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
available 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
implementation. 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 results in a smooth transition from constant voltage to
constant current mode when current limit is detected, reduc-
ing the frequency and output voltage, without the use of
foldback. Additional features include: VCC under-voltage
lockout, thermal shutdown, gate drive under-voltage lockout,
and maximum duty cycle limiter.
AEC-Q100 Grade 1 qualified (-40°c to 125°c)
■
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Maximum switching frequency: 2.6 MHz
(VIN=14V,Vo=3.3V)
Input Voltage Range: 6V to 40V
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Integrated N-Channel buck switch
Integrated start-up regulator
No loop compensation required
Ultra-Fast transient response
Operating frequency remains constant with load current
and input voltage
Maximum Duty Cycle Limited During Start-Up
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■
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Adjustable output voltage
Valley Current Limit At 0.64A
Precision internal reference
Low bias current
Highly efficient operation
Thermal shutdown
Typical Applications
Automotive Safety and Infotainment
■
■
■
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High Efficiency Point-Of-Load (POL) Regulator
Non-Isolated Telecommunication Buck Regulator
Secondary High Voltage Post Regulator
Package
micro SMD (2mm x 2mm)
■
Basic Step Down Regulator
30102731
© 2010 National Semiconductor Corporation
301027
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Connection Diagrams
30102733
Top View
30102702
Bump Side
Ordering Information
NSC
Package
Drawing
Junction
Temperature Range
Order Number
Package Type
Supplied As
Feature
250 Units on Tape
and Reel
AEC-Q100 Grade 1
qualitifed. Automotive
Grade Production
Flow. *
LM34919BQTL
LM34919BQTLX
LM34919BTL
10-Bump micro SMD
10–Bump micro SMD
10-Bump micro SMD
10–Bump micro SMD
TLP10KAA
TLP10KAA
TLP10KAA
TLP10KAA
−40°C to + 125°C
−40°C to + 125°C
3000 Units on
Tape and Reel
250 Units on Tape
and Reel
3000 Units on
Tape and Reel
LM34919BTLX
For detailed information on micro SMD packages, refer to the Application Note AN-1112.
*Automotive grade (Q) product incorporates enhanced manufacturing and support processes for the automotive market, includng
defect detection methodologies. Reliability qualification is compliant with the requirements and temperature grades defined in the
AEC-Q100 standard. Automotive grade products are identified with the letter Q. For more information go to http://www.national.com/
automotive.
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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 regulated 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 6.0V 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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SGND to RTN
SS, RON/SD to RTN
FB to RTN
Storage Temperature Range
For soldering specs see:
www.national.com/ms/MS/MS-SOLDERING.pdf
JunctionTemperature 150°C
-0.3V to +0.3V
-0.3V to 4V
-0.3 to 7V
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
-65°C to +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
BST to SW
VCC to RTN
-1.5V to 44V
Operating Ratings (Note 1)
VIN
2kV
44V
14V
14V
6.0V to 40V
−40°C to + 125°C
Junction Temperature
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 = 20kΩ. See (Note 4).
Symbol
Parameter
Conditions
Min
Typ
Max
Units
Start-Up Regulator, VCC
VCCReg
VCC regulated output
VIN = 12V
6.6
5.3
7
7.4
V
VIN =6V, ICC = 3 mA,
5.91
20
VIN-VCC dropout voltage
VCC Output Impedance
ICC = 0 mA, non-switching
VCC = UVLOVCC + 250 mV
mV
24
12
0 mA ≤ ICC ≤ 5 mA, VIN = 6V
Ω
0 mA ≤ ICC ≤ 5 mA, VIN = 8V
VCC current limit (Note 3)
VCC = 0V
15
5.25
5.1
mA
V
UVLOVCC VCC under-voltage lockout threshold VCC increasing
measured at VCC
VCC decreasing
5.25
V
UVLOVCC hysteresis, at VCC
150
5.25
5.1
mV
V
VCC under-voltage lock-out threshold VIN increasing, ICC = 3 mA
5.6
5.4
measured at VIN
VIN decreasing, ICC = 3 mA
100 mV overdrive
V
UVLOVCC filter delay
IIN operating current
IIN shutdown current
3
µs
mA
µA
IQ
Non-switching, FB = 3V, SW = Open
RON/SD = 0V, SW = Open
0.78
215
1.0
ISD
330
Switch Characteristics
Rds(on)
UVLOGD
Buck Switch Rds(on)
ITEST = 200 mA
0.5
3.6
3.2
400
1.0
Ω
V
Gate Drive UVLO
VBST - VSW Increasing
VBST - VSW Decreasing
2.65
4.40
UVLOGD hysteresis
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
135
50
0.76
A
Resistance from ISEN to SGND
Response time
mΩ
ns
On Timer
tON - 1
On-time
127
0.4
170
110
335
0.74
213
1.2
ns
ns
ns
V
VIN = 12V, RON = 20kΩ
VIN = 24V, RON = 20 kΩ
VIN = 6V, RON = 20 kΩ
Voltage at RON/SD rising
tON - 2
tON - 3
On-time
On-time
Shutdown threshold
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Symbol
Parameter
Threshold hysteresis
Conditions
Voltage at RON/SD
Min
Typ
Max
Units
40
mV
Off Timer
tOFF
Minimum Off-time
VIN = 6V, ICC = 3mA
VIN = 8V, ICC = 3mA
60
58
88
82
120
118
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
Thermal Shutdown
TSD Thermal shutdown temperature
Thermal shutdown hysteresis
Thermal Resistance
Junction to Ambient
0 LFPM Air Flow
175
20
°C
°C
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: Typical specifications represent the most likely parametric norm at 25°C operation.
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Typical Performance Characteristics
Efficiency at 2.1 MHz, 3.3V
Efficiency at 250 kHz, 3.3V
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Efficiency at 2.1 MHz, 5V
VCC vs. VIN
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VCC vs. ICC
ICC vs. Externally Applied VCC
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ON-TIME vs. VIN and RON
Voltage at the RON/SD Pin
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Operating Current into VIN
Shutdown Current into VIN
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VCC UVLO at Vin vs. Temperature
Gate Drive UVLO vs. Temperature
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VCC Voltage vs. Temperature
VCC Output Impedance vs. Temperature
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VCC Current Limit vs. Temperature
Reference Voltage vs. Temperature
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Soft-Start Current vs. Temperature
On-Time vs. Temperature
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Minimum Off-Time vs. Temperature
Current Limit Threshold vs. Temperature
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Operating & Shutdown Current vs. Temperature
RON Pin Shutdown Threshold vs. Temperature
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Block Diagram
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30102734
FIGURE 1. Start Up Sequence
Functional Description
Control Circuit Overview
The LM34919B 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 LM34919B 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 LM34919B operates in continuous
conduction mode at heavy load currents and discontinuous
conduction mode at light load currents. In continuous con-
duction mode current always flows through the inductor, nev-
er reaching zero during the off-time. In this mode the
operating frequency remains relatively constant with load and
line variations. The minimum load current for continuous con-
The LM34919B can be applied in numerous applications to
efficiently 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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duction mode is one-half the inductor’s ripple current ampli-
tude. The operating frequency is approximately:
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 LM34919B 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 LM34919B. 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 15 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 5.25V, 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 VCC UVLO
falling threshold (≊5.1V). When VCC falls below the falling
threshold the VCC UVLO activates to shut off the output. If
VCC is externally loaded, the minimum input voltage increas-
es.
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:
VOUT = 2.5 x (R1 + R2) / R2
30102711
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 LM34919B 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. The minimum off-time limits the maxi-
mum duty cycle achievable with a low voltage at VIN. At high
values of VIN, the minimum on-time is limited to ≊ 90 ns.
The LM34919B can be remotely shut down by taking the
RON/SD pin low. 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 5.0V, depending on VIN and the RON resistor.
30102713
FIGURE 3. Shutdown Implementation
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30102714
FIGURE 4. Inductor Current - Current Limit Operation
N - Channel Buck Switch and Driver
Applications Information
The LM34919B 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 LM34919B
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 = 3.3V
- VIN = 6V to 24V
- Minimum load current = 200 mA
- Maximum load current = 600 mA
- Switching Frequency = 1.5 MHz
- 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 = 0.32. 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 R2 and 787Ω for R1.
RON: This resistor sets the on-time, and (by default) the
switching frequency. The switching frequncy must be less
than 1.53 MHz to ensure the minimum forced on-time does
not interfere with the circuit's proper operation at the maxi-
mum input voltage. 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 LM34919B should be operated so the junction tempera-
ture does not exceed 125°C. If the junction temperature in-
creases, an internal Thermal Shutdown circuit, which acti-
vates (typically) at 175°C, takes the controller to a low power
reset state by disabling the buck switch. This feature helps
prevent catastrophic failures from accidental device over-
heating. 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 90 ns at maximum VIN.
A standard value 28 kΩ resistor is used, resulting in a nominal
frequency of 1.49 MHz. The minimum on-time is ≊129 ns at
Vin = 24V, and the maximum on-time is ≊424 ns at Vin = 6V.
Alternately, RON can be determined using Equation 4 if a spe-
cific 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 LM34919B, 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 more than 0.5V. The
minimum 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). 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 8.2 µH inductor is selected. The maximum
ripple amplitude, which occurs at maximum VIN, calculates to
325 mA p-p, and the peak current is 763 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 LM34919B 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 0.27Ω 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 780 mA at 6V, and 812 mA at 24V.
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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30102721
FIGURE 5. Example Circuit
30102740
FIGURE 6. Efficiency (Circuit of Figure 5)
30102723
FIGURE 7. Frequency vs. VIN (Circuit of Figure 5)
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16
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:
30102727
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.
30102725
30102728
FIGURE 8. Reduced Ripple Configuration
FIGURE 10. Alternate Minimum Output 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 LM34919B 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 either shutdown, or cycle on and off at a low frequency. If
the load current is expected to drop below 1 mA in the appli-
cation, R1 and R2 should be chosen low enough in value so
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.
they provide the minimum required current at nominal VOUT
.
PC BOARD LAYOUT
Refer to application note AN-1112 for PC board guidelines for
the Micro SMD package.
The LM34919B regulation, over-voltage, and current limit
comparators are very fast, and respond to short duration
noise pulses. Layout considerations are therefore critical for
optimum performance. The layout must be as neat and com-
pact 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 50 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 LM34919B 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:
17
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PD1 = Iout x VF x (1-D)
internal dissipation of the LM34919B will produce excessive
junction temperatures during normal operation, good use of
the PC board’s ground plane can help to dissipate heat. Ad-
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 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:
PL1 = Iout2 x RL x 1.1
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
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18
Physical Dimensions inches (millimeters) unless otherwise noted
Note: X1 = 1.753 mm, ±0.030 mm
X2 = 1.987 mm, ±0.030 mm
X3 = 0.60 mm, ±0.075 mm
10 Bump micro SMD Package
NS Package Number TLP10KAA
19
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Notes
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