LM34917ATL/NOPB [TI]

具有智能限流功能的 2.3x2mm、8-33V、1.25A、恒定导通时间非同步降压稳压器 | YZR | 12 | -40 to 125;


型号：	LM34917ATL/NOPB
厂家：	TEXAS INSTRUMENTS
描述：	具有智能限流功能的 2.3x2mm、8-33V、1.25A、恒定导通时间非同步降压稳压器 \| YZR \| 12 \| -40 to 125 稳压器
文件：	总22页 (文件大小：757K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LM34917A

www.ti.com.cn

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

具有智能电流限制的 LM34917A 超小型 33V，1.25A 恒定接通时间降压开

关稳压器

查询样品: LM34917A

特性

封装

•

工作输入电压范围：8V 至 33V

•

12 焊锡凸点 DSBGA 封装

•

芯片尺寸球栅阵列 (DSBGA) 封装

≊35V 时，输入过压关断

瞬态能力达到 50V

说明

LM34917A 降压开关稳压器特有执行低成本、高效降

压偏置稳压器（至少能够为负载提供 1.25A 的电流）

所需的全部功能。为了减少由可能的电感器饱和所导

致的过多开关电流，谷值电流限值阀值随着输入和输出

电压的变化而改变，而且接通时间会在检测到电流限值

时被减少。这个降压稳压器包含一个 N 通道降压开关

并且采用 12 引脚 DSBGA 封装。恒定接通时间反馈

调节机制无需环路补偿，从而实现快速负载瞬态响应，

并简化电路实现。由于输入电压和接通时间之间的反

比关系，线路和负载变化时，运行频率保持恒定。谷

值电流限制可在检测到电流限值时实现恒定电压到恒定

电流的平滑转换，从而在不使用折返的情况下减少频率

和输出电压。额外特性包括：V_CC欠压闭锁、输入过压

关断、热关断、栅极驱动欠压闭锁和最大占空比限制。

集成 N 通道降压开关

随 V_输入和 V_输出变化的谷值电流限值以减少过多电

感器电流

•

电流限制时，接通时间被减少

集成启动稳压器

无需环路补偿

超快瞬态响应

最大开关频率：2MHz

在负载电流和输入电压变化时，工作频率几乎保持

恒定

•

可编程软启动

精密内部基准

可调输出电压

热关断

典型应用

•

高效负载点 (POL) 稳压器

非隔离式降压稳压器

次级高压后置稳压器

基本降压稳压器

8V - 33V

Input

VIN

VCC

LM34917A

BST

RON/SD

OUT

SHUT

DOWN

ISEN

RTN

SGND

Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of

Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

All trademarks are the property of their respective owners.

PRODUCTION DATA information is current as of publication date.

Products conform to specifications per the terms of the Texas

Instruments standard warranty. Production processing does not

necessarily include testing of all parameters.

English Data Sheet: SNOSAX7

LM34917A

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

www.ti.com.cn

连接图

BST

VIN

VCC

VIN

ISEN

RON

SGND

RTN

图 1. 焊锡凸块一侧

封装编号 YZR0012UNA

图 2. 顶视图

封装编号 YZR0012UNA

引脚说明

引脚编号

名称

说明

应用信息

SGND

RTN

感测接地

电路接地

再循环电流从这个引脚流入电流感测电阻器。

针对除电流限值检测之外所有内部电路的接地。

反馈输入来自经稳压输出

电流感测

被内部连接至稳压和过压比较器。此稳压电平为 2.5V。

ISEN

再循环电流从这个引脚流入续流二极管。

RON/SD

接通时间控制和关断

VIN 和这个引脚之间的一个外部电阻器设定降压开关接通时

间。把这个引脚接地将关断稳压器。

C1，C2

VIN

VCC

软启动

一个内部电流源将外部电容器充电至 2.5V，从而提供软启动功

能。

输入电源电压

启动稳压器的输出

工作输入电压范围为 8.0V 至 33V，此时过压关断在内部设定

为 ≊35V。瞬态能力为 50V。

标称稳压值为 7.0V。将一个 0.1µF 电容器由这个引脚连接至

RTN。可将一个外部电压（8V 至 14V）施加到这个引脚来减

少内部耗散。将一个内部二极管连接在 VCC 和 VIN 之间。

D1，D2

开关节点

内部连接至降压开关源。连接至电感器、二极管和引导电容

器。

BST

针对引导电容器的升压引脚

将一个 0.022µF 电容器由 SW 连接至这个引脚。每次关闭时

间时，通过一个内部二极管对此电容器充电。

These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam

during storage or handling to prevent electrostatic damage to the MOS gates.

LM34917A

www.ti.com.cn

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

Absolute Maximum Ratings⁽¹⁾⁽²⁾

VIN to RTN

50V

BST to RTN

64V

SW to RTN (Steady State)

BST to VCC

-1.5V

50V

VIN to SW

50V

BST to SW

14V

VCC to RTN

14V

SGND to RTN

-0.3V to +0.3V

See text

-0.3V to 4V

-0.3 to 7V

Current out of ISEN

SS to RTN

All Other Inputs to RTN

(3)

ESD Rating

Human Body Model

Storage Temperature Range

Junction Temperature

2kV

-65°C to +150°C

150°C

(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 specifications and test conditions, see the Electrical Characteristics.

(2) If Military/Aerospace specified devices are required, please contact the TI Sales Office/Distributors for availability and specifications.

(3) The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin.

(1)

Operating Ratings

V_INVoltage

8.0V to 33V

−40°C to + 125°C

Junction Temperature

(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 specifications and test conditions, see the Electrical Characteristics.

Electrical Characteristics

Limits in standard type are for T_J= 25°C only; limits in boldface type apply over the junction temperature (T_J) range of -40°C

to +125°C. Minimum and Maximum limits are specified through test, design, or statistical correlation. Typical values represent

the most likely parametric norm at T_J= 25°C, and are provided for reference purposes only. Unless otherwise stated the

(2)

following conditions apply: V_IN= 12V, R_ON= 200kΩ. See ⁽¹⁾and

Symbol

Parameter

Conditions

Min

6.6

Typ

Max

7.4

Units

Start-Up Regulator, V_CC

V_CCReg

V_CCregulated output

Vin > 9V

I_CC= 0 mA,

7.0

1.3

V_IN-V_CCdropout voltage

V_CC= UVLO_VCC+ 250 mV

V_CCoutput impedance

(0 mA ≤ I_CC≤ 5 mA)

V_IN= 8V

150

0.75

Ω

V_IN= 12V

(3)

V_CCcurrent limit

V_CC= 0V

UVLO_VCC

V_CCunder-voltage lockout

threshold

V_CCincreasing

5.45

UVLO_VCChysteresis

UVLO_VCCfilter delay

I_INoperating current

I_INshutdown current

V_CCdecreasing

145

µs

100 mV overdrive

Non-switching, FB = 3V

RON/SD = 0V

0.68

0.95

160

µA

Switch Characteristics

Rds(on)

UVLO_GD

Buck Switch Rds(on)

Gate Drive UVLO

I_TEST= 200 mA

0.33

0.7

Ω

V_BST- V_SWIncreasing

2.65

4.62

(1) For detailed information on soldering DSBGA packages, refer to Application Note AN-1112 (SNVA009).

(2) Typical specifications represent the most likely parametric norm at 25°C operation.

(3) V_CCprovides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading

LM34917A

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

www.ti.com.cn

Electrical Characteristics (continued)

Limits in standard type are for T_J= 25°C only; limits in boldface type apply over the junction temperature (T_J) range of -40°C

to +125°C. Minimum and Maximum limits are specified through test, design, or statistical correlation. Typical values represent

the most likely parametric norm at T_J= 25°C, and are provided for reference purposes only. Unless otherwise stated the

following conditions apply: V_IN= 12V, R_ON= 200kΩ. See ⁽¹⁾and ⁽²⁾

Symbol

Parameter

Conditions

Min

Typ

Max

Units

UVLO_GDhysteresis

450

Softstart Pin

V_SS

I_SS

Pull-up voltage

2.5

µA

Internal current source

11.6

0.18

V_RES

Restart threshold after OVP

shutdown

Current Limit

I_LIM

Threshold

V_IN= 8V, V_FB= 2.4V

1.15

1.05

0.95

1.35

1.2

1.55

1.45

1.35

V_IN= 30V, V_FB= 2.4V

V_IN= 30V, V_FB= 1.0V

1.15

150

Response time

On Timer

t_ON- 1

On-time (normal operation)

On-time (current limit)

V_IN= 10V, R_ON= 200 kΩ

V_IN= 32V, R_ON= 200 kΩ

V_IN= 10V, R_ON= 200 kΩ

2.1

0.3

2.8

860

1.13

0.65

3.5

1.0

µs

t_ON- 2

t_ON- 3

Shutdown threshold at RON/SD Voltage at RON/SD rising

Shutdown Threshold hysteresis Voltage at RON/SD falling

Off Timer

t_OFF

Minimum Off-time

Regulation and Over-Voltage Comparators (FB Pin)

V_REF

FB regulation threshold

FB over-voltage threshold

FB bias current

SS pin = steady state

2.445

33.0

2.50

2.9

2.550

36.9

FB = 3V

Input Over-Voltage Shutdown

V_IN(OV)Shutdown voltage threshold at

VIN

Thermal Shutdown

T_SD

V_INincreasing

34.8

Thermal shutdown temperature Junction temperature rising

Thermal shutdown hysteresis

175

°C

Thermal Resistance

θ_JAJunction to Ambient

0 LFPM Air Flow

°C/W

LM34917A

www.ti.com.cn

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

Typical Performance Characteristics

Unless otherwise specified the following conditions apply: T_J= 25°C

Efficiency at 1.5 MHz

Efficiency at 2 MHz

Figure 3.

Figure 4.

V_CCvs. V_IN

ON-Time vs. V_INand R_ON

7.5

= 600 kW

200 kW

100 kW

7.0

6.5

6.0

5.5

5.0

= 94 kHz

3.0

1.0

= 350 kHz

400 kW

= 700 kHz

0.3

0.1

50 kW

(V)

6.5

7.0

7.5

8.0

(V)

8.5

9.0

Figure 5.

Figure 6.

Voltage at the R_ON/SD Pin

Valley Current Limit Threshold vs. V_FBand V_IN

1.5

3.0

2.0

1.0

1.4

= 50k

100k

200k

= 8V

1.3

1.2

1.1

1.0

15V

24V

600k

33V

0.5

1.0

1.5

(V)

2.0

2.5

(V)

Figure 7.

Figure 8.

LM34917A

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

www.ti.com.cn

Typical Performance Characteristics (continued)

Unless otherwise specified the following conditions apply: T_J= 25°C

V_CCvs. I_CC

I_CCvs Externally Applied V_CC

8 10V

= 8V

700 kHz

350 kHz

= 9V

Externally Loaded

= 94 kHz

= 350 kHz

(V)

(mA)

APPLIED V

Figure 9.

Figure 10.

Shutdown and Operating Current Into V_IN

Figure 11.

LM34917A

www.ti.com.cn

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

Typical Application Circuit and Block Diagram

7V START-UP

REGULATOR

LM34917A

Input

VIN

VCC

8V - 33V

UVLO

0V SHUTDOWN

34.8V

ON TIMER

GND

0.65V

MINIMUM

OFF TIMER

START

D Ton FINISH

RON/SD

FINISH

START

BST

Gate Drive

UVLO

THERMAL

SHUTDOWN

2.5V

11.6 mA

LOGIC

LEVEL

SHIFT

Driver

REGULATION

COMPARATOR

OUT

OVER-VOLTAGE

COMPARATOR

CURRENT LIMIT

COMPARATOR

2.9V

RTN

ISEN

SENSE

Threshold

Adjust

41 mW

SGND

LM34917A

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

www.ti.com.cn

7.0V

UVLO

SW Pin

Inductor

Current

2.5V

SS Pin

OUT

Figure 12. Startup Sequence

LM34917A

www.ti.com.cn

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

FUNCTIONAL DESCRIPTION

The LM34917A Step Down Switching Regulator features all the functions needed to implement a low cost,

efficient buck bias power converter capable of supplying at least 1.25A to the load. This high voltage regulator

contains an N-Channel buck switch, is easy to implement, and is available in the DSBGA package. The

regulator’s operation is based on a constant on-time control scheme where the on-time is inversely proportional

to the input voltage. This feature results in the operating frequency remaining relatively constant with load and

input voltage variations. The feedback control scheme requires no loop compensation resulting in very fast load

transient response. The valley current limit scheme protects against excessively high currents if the output is

short circuited when V_INis high. To aid in controlling excessive switch current due to a possible saturating

inductor the valley current limit threshold changes with input and output voltages, and the on-time is reduced by

approximately 50% when current limit is detected. An over-voltage detection at V_INstops the circuit's switching

when the input voltage exceeds 34.8V. The LM34917A can be applied in numerous applications to efficiently

regulate down higher voltages. Additional features include: Thermal shutdown, V_CCunder-voltage lock-out, gate

drive under-voltage lock-out, and maximum duty cycle limit.

Control Circuit Overview

The LM34917A 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 reference (2.5V). If the FB voltage is below

the reference the buck switch is switched on for a time period determined by the input voltage and a

programming resistor (R_ON). Following the on-time the switch remains off until the FB voltage falls below the

reference, but for a time not less than the minimum off-time forced by the LM34917A. The buck switch is then

switched on for another on-time period.

When in regulation, the LM34917A operates in continuous conduction mode at heavy load currents and

discontinuous conduction mode at light load currents. In continuous conduction mode the inductor’s current is

always greater than zero, and the operating frequency remains relatively constant with load and line variations.

The minimum load current for continuous conduction mode is one-half the inductor’s ripple current amplitude.

The approximate operating frequency is calculated as follows:

V_OUTx (V_INœ 1.35V)

V_INx 1.16 x 10^-10x (R_ON+ 1.4k)

f_SW

(1)

The buck switch duty cycle is equal to:

V_OUT

V_IN

t_ON

DC =

= t_ONx f_SW

t_ON+ t_OFF

(2)

In discontinuous conduction mode, where the inductor’s current reaches zero during the off-time forcing a longer-

than-normal off-time, 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 losses reduce with the reduction in

load and frequency. The approximate discontinuous operating frequency can be calculated as follows:

V_OUT²x L1 x 1.48 x 10²⁰

f_SW

R_Lx R_ON

(3)

where R_L= the load resistance, and L1 is the circuit’s inductor.

The output voltage is set by the two feedback resistors (R1, R2 in the Block Diagram). The regulated output

voltage is calculated as follows:

V_OUT= 2.5 x (R1 + R2) / R2

(4)

Output voltage regulation is based on supplying ripple voltage to the feedback input (FB pin) in phase with the

SW pin. The LM34917A requires a minimum of 25 mVp-p of ripple voltage at the FB pin. The ripple is generated

as a triangle wavefrom at the junction of R3 and C8 as the SW pin switches high and low, and fed to the FB pin

by C7.

If the voltage at FB rises above 2.9V, due to a transient at V_OUTor excessive inductor current which creates

higher than normal ripple at V_OUT, the internal over-voltage comparator immediately shuts off the internal buck

switch. The next on-time starts when the voltage at FB falls below 2.5V and the inductor current falls below the

current limits threshold.

LM34917A

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

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ON-Time Timer

The on-time for the LM34917A is determined by the R_ONresistor and the input voltage (V_IN), calculated from:

1.16 x 10^-10x (R_ON+ 1.4 kW)

t_ON

+ 100 ns

V_IN- 1.35V

(5)

The inverse relationship with V_INresults in a nearly constant frequency as V_INis varied. To set a specific

continuous conduction mode switching frequency (f_SW), the R_ONresistor is determined from the following:

V_OUTx (V_IN- 1.35V)

V_INx 1.16 x 10^-10x f_SW

-1.4kW

R_ON

(6)

Equation 1, Equation 5 and Equation 6 are valid only during normal operation - i.e., the circuit is not in current

limit. When the LM34917A operates in current limit, the on-time is reduced by approximately 50%. This feature

reduces the peak inductor current which may be excessively high if the load current and the input voltage are

simultaneously high. This feature operates on a cycle-by-cycle basis until the load current is reduced and the

output voltage resumes its normal regulated value. Equation 1, Equation 5 and Equation 6 have a ±25%

tolerance.

Remote Shutdown

The LM34917A can be remotely shut down by taking the RON/SD pin below 0.65V. See Figure 13. In this mode

the SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the RON/SD

pin allows the circuit to resume operation after the SS pin voltage is below 0.18V. The voltage at the RON/SD pin

is normally between 1.4V and 3.5V, depending on V_INand the R_ONresistor.

Input

Voltage

LM34917A

RON/SD

STOP

RUN

Figure 13. Remote Shutdown

Input Over-Voltage Shutdown

If the input voltage at VIN increases above 34.8V an internal comparator disables the buck switch and the on-

timer, and grounds the soft-start pin. Normal operation resumes when the V_INvoltage reduces below 34.8V, and

when the soft-start voltage (at the SS pin) has reduced below 0.18V.

Current Limit

Current limit detection occurs during the off-time by monitoring the recirculating current flowing out of the ISEN

pin. Referring to the Block Diagram, during the off-time the inductor current flows through the load, into SGND,

through the internal sense resistor, out of ISEN and through D1 to the inductor. If that current exceeds the

current limit threshold the current limit comparator output delays the start of the next on-time period. The next on-

time starts when the current out of ISEN is below the threshold and the voltage at FB falls below 2.5V. The

operating frequency is typically lower due to longer-than-normal off-times.

The valley current limit threshold is a function of the input voltage (V_IN) and the output voltage sensed at FB, as

shown in the graph “Valley Current Limit Threshold vs. V_FBand V_IN”. This feature reduces the inductor current’s

peak value at high line and load. To further reduce the inductor’s peak current, the next cycle’s on-time is

reduced by approximately 50% if the voltage at FB is below its threshold when the inductor current reduces to

the current limit threshold (V_OUTis low due to current limiting).

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Figure 14 illustrates the inductor current waveform during normal operation and in current limit. During the first

“Normal Operation” the load current is I_OUT1, the average of the ripple waveform. As the load resistance is

reduced, the inductor current increases until it exceeds the current limit threshold. During the “Current Limited”

portion of Figure 14, the current limit threshold lowers since the high load current causes V_OUT(and the voltage

at FB) to reduce. The on-time is reduced by approximately 50%, resulting in lower ripple amplitude for the

inductor’s current. During this time the LM34917A is in a constant current mode, with an average load current

equal to the current limit threshold + ΔI/2 (I_OUT2). Normal operation resumes when the load current is reduced to

I_OUT3, allowing V_OUT, the current limit threshold, and the on-time to return to their normal values. Note that in the

second period of “Normal Operation”, even though the inductor’s peak current exceeds the current limit threshold

during part of each cycle, the circuit is not in current limit since the current falls below the threshold before the

feedback voltage reduces to its threshold to initiate the next on-time.

The peak current allowed through the buck switch, and the ISEN pin, is 2A, and the maximum allowed average

current is 1.5A.

OUT2

Current Limit

Threshold

OUT3

OUT1

2.5V

Normal

Operation

Current

Limited

Normal

Operation

Load

Current

Increases

Load Current

Decreases

Figure 14. Inductor Current - Normal and Current Limit Operation

N - Channel Buck Switch and Driver

The LM34917A integrates an N-Channel buck switch and associated floating high voltage gate driver. The gate

driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.022

µF capacitor (C4) connected between BST and SW provides the voltage to the driver during the on-time. During

each off-time, the SW pin is at approximately -1V, and C4 is recharged for the next on-time from V_CCthrough the

internal diode. The minimum off-time ensures a minimum time each cycle to recharge the bootstrap capacitor.

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 V_CCreaches the under-voltage threshold, an internal

11.6 µA current source charges up the external capacitor at the SS pin to 2.5V (t₂in Figure 12). The ramping

voltage at SS (and the non-inverting input of the regulation comparator) ramps up the output voltage in a

controlled manner.

An internal switch grounds the SS pin if V_CCis below the under-voltage lockout threshold, if the RON/SD pin is

grounded, or if V_INexceeds the overvoltage threshold.

LM34917A

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

www.ti.com.cn

Thermal Shutdown

The LM34917A should be operated so the junction temperature does not exceed 125°C. If the junction

temperature increases above that, an internal Thermal Shutdown circuit activates (typically) at 175°C, taking the

controller to a low power reset state by disabling the buck switch. This feature helps prevent catastrophic failures

from accidental device overheating. When the junction temperature reduces below 155°C (typical hysteresis =

20°C), normal operation resumes.

Applications Information

EXTERNAL COMPONENTS

The procedure for calculating the external components is illustrated with the following design example. Referring

to the Block Diagram, the circuit is to be configured for the following specifications:

•

V_OUT= 5V

V_IN= 8V to 33V

Minimum load current = 200 mA

Maximum load current = 1000 mA

Switching Frequency = 1.5 MHz

Soft-start time = 5 ms

Output voltage ripple level: Minimum

R1 and R2: These resistors set the output voltage. The ratio of the feedback resistors is calculated from:

R1/R2 = (V_OUT/2.5V) - 1

(7)

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, 2.49 kΩ is chosen for R1 and R2.

R_ON: This resistor sets the on-time, and (by default) the switching frequency. Since the maximum frequency is

limited by the minimum off-time forced by the LM34917A, first check that the desired frequency is less than:

V_IN- V_OUT

= 3.57 MHz at V_IN= 8V

f_SW

V_INx 105 ns

(8)

The R_ONresistor is calculated from Equation 6 using the minimum input voltage:

V_OUTx (V_IN(min)- 1.35V)

V_IN(min)x 1.16 x 10^-10x f_SW

- 1.4 kW = 22.49 kW

R_ON

(9)

Equation 5 is used to verify that this value resistor does not set an on-time less than 120 ns at maximum input

voltage. A standard value 22.1 kΩ resistor is used, resulting in a nominal frequency of 1.49 MHz. The minimum

on-time is 188 ns at Vin = 33V, and the maximum on-time is 510 ns at Vin = 8V.

L1: The main parameter affected by the inductor is the inductor current ripple amplitude (I_OR). The minimum load

current 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 LM34917A, but serves as a

guideline for selecting L1. For this example, the maximum ripple current should be less than:

I_OR(MAX)= 2 x I_OUT(min)= 400 mAp-p

(10)

For other applications, if the minimum load current is zero, use 20% of I_OUT(max)for I_OUT(min)in Equation 10. The

ripple amplitude calculated in Equation 10 is then used in the following equation:

t_on(min)x (V_IN(max)œ V_OUT

)

= 13.2 mH

L1_(min)

I_OR(max)

(11)

A standard value 15 µH inductor is selected. The maximum ripple amplitude, which occurs at maximum V_IN,

calculates to 351 mA p-p, and the peak current is 1175 mA at maximum load current. Ensure the selected

inductor is rated for this peak current.

C2: C2 should typically 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.

LM34917A

www.ti.com.cn

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

C1 and C5: C1’s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple

at V_IN, since it is assumed the voltage source feeding VIN has some amount of source impedance.

At maximum load current, when the buck switch turns on, the current into V_INsuddenly increases to the lower

peak of the 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 V_INdrop below ≊7.5V. The minimum value for

C1 is calculated from:

I_{OUT (max)}x t_ON

C1 =

= 1.02 mF

(12)

where t_ONis the maximum on-time, and ΔV is the allowable ripple voltage at V_IN(0.5V at V_IN= 8V). C5’s purpose

is to minimize transients and ringing due to long lead inductance leading the VIN pin. A low ESR 0.1 µF ceramic

chip capacitor must be located close to the VIN and RTN pins.

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 capacitor. C3’s value, and the VCC current

limit, determine a portion of the turn-on-time (t1 in Figure 12).

C4: The recommended value for C4 is 0.022 µF. A high quality ceramic capacitor with low ESR is recommended

as C4 supplies 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 soft-start time, i.e. the time for the output voltage to reach its final

value (t₂in Figure 12). The capacitor value is determined from:

t₂x 11.6 mA

2.5V

C6 =

= 0.023 mF

(13)

R3, C7, C8: The ripple amplitude at V_OUTis determined by C2’s characteristics and the inductor’s ripple current

amplitude, and typically ranges from 5 mV to 30 mV over the Vin range. Since the LM34917A’s regulation

comparator requires a minimum of 25 mVp-p ripple at the FB pin, these three components are added to generate

and provide the necessary ripple to FB in phase with the waveform at SW. R3 and C8 are chosen to generate a

sawtooth waveform at their junction, and that voltage is AC coupled to the FB pin via C7. To determine the

values for R3, C7 and C8, the following procedure is used:

Calculate V_A= V_OUT– (V_SWx (1 – (V_OUT/V_IN(min)))

(14)

where V_SWis the absolute value of the voltage at the SW pin during the off-time (typically 1V). V_A, the DC

voltage at the R3/C8 junction, calculates to 4.63V, and is used in the next equation.

(V_{IN (min)}- V_A) x t_ON

= 17.5 X 10^-6

R3 x C8 =

(15)

where t_ONis the maximum on-time (at minimum input voltage), and ΔV is the desired ripple amplitude at the

R3/C8 junction, typically 100 mV. R3 and C8 are chosen from standard value components to satisfy the above

product. For this example, 3300 pF is chosen for C8, and 5.23 kΩ is chosen for R3. C7 is chosen large

compared to C8, typically 0.1 µF.

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

must be rated for the maximum input voltage, the maximum load current, and the peak current which occurs

when the current limit and maximum ripple current are reached simultaneously. The diode’s average power

dissipation is calculated from:

P_D1= V_Fx I_OUTx (1-D)

(16)

where V_Fis the diode’s forward voltage drop, and D is the on-time duty cycle.

FINAL CIRCUIT

The final circuit is shown in Figure 15, and its performance is shown in Figure 16 and Figure 17. Current limit

measured approximately 1.34A at Vin = 8V, and 1.27A at Vin = 33V. The output ripple amplitude measured 4

mVp-p at Vin = 8V, and 14 mVp-p at Vin = 33V.

LM34917A

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

www.ti.com.cn

8V to 33V

Input

VCC

VIN

0.1mF

0.1 mF

2 mF

LM34917A

BST

0.022 mF

15 mH

V_OUT

22.1 kW

RON/SD

2.49 kW

3300 pF

0.1mF

5.23 kW

SHUT

DOWN

ISEN

20 mF

0.022 mF

2.49 kW

SGND

RTN

Figure 15. Example Circuit

Figure 16. Efficiency vs. Load Current and V_IN(Circuit of Figure 15)

Figure 17. Frequency vs. V_IN(Circuit of Figure 15)

ALTERNATE OUTPUT RIPPLE CONFIGURATIONS

For applications which can accept higher levels of ripple at V_OUT, the following configurations are simpler and a

bit more economical.

a) Alternate #1: In Figure 18, R3, C7 and C8 are removed, and Cff and R4 are installed, resulting in a higher

ripple level than the circuit of Figure 15. Ripple is created at V_OUTby the inductor’s ripple current passing through

R4. That ripple voltage is AC coupled to the FB pin through Cff, allowing the minimum ripple at V_OUTto be set at

25 mVp-p. The minimum ripple current amplitude (I_OR(min)) is calculated by re-arranging Equation 11 using t_ON(max)

and V_IN(min). The minimum value for R4 is calculated from:

25 mV

R4 =

I_{OR (min)}

(17)

LM34917A

www.ti.com.cn

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

The next larger standard value resistor should be selected for R4 to allow for tolerances. The minimum value for

Cff is determined from:

t_{ON (max)}

Cff =

(R1//R2)

(18)

The next larger standard value capacitor should be used for Cff.

OUT

Cff

LM34917A

Figure 18. Reduced Ripple Configuration

b) Alternate #2: In Figure 19, R3, C7 and C8 are removed, and R4 is installed, resulting in a higher ripple level

than the circuits of Figure 15 and Figure 18. Ripple is created at V_OUTby the inductor’s ripple current passing

through R4. That ripple voltage is coupled to the FB pin through the feedback resistors (R1, R2). Since the

LM34917A requires a minimum of 25 mVp-p ripple at the FB pin, the ripple required at V_OUTis higher than 25

mVp-p by the gain of the feedback resistors. The minimum ripple current (I_OR(min)) is calculated by re-arranging

Equation 11 using t_ON(max)and V_IN(min). The minimum value for R4 is calculated from:

25 mV x (R1 + R2)

R4_(min)

R2 x I_{OR (min)}

(19)

The next larger standard value resistor should be used for R4.

OUT

LM34917A

Figure 19. Maximum Ripple Configuration

c) Alternate minimum ripple configuration: The circuit in Figure 20 is the same as that in Figure 19, except

the output voltage is taken from the junction of R4 and C2. The ripple at V_OUTis determined by the inductor’s

ripple current and C2’s characteristics. However, R4 slightly degrades the load regulation. This circuit may be

suitable if the load current is fairly constant. R4 is calculated as described in Alternate #2 above.

LM34917A

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

www.ti.com.cn

LM34917A

OUT

Figure 20. Alternate Minimum Output Ripple Configuration

Minimum Load Current

The LM34917A 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 application, R1 and R2 should be

chosen low enough in value so they provide the minimum required current at nominal V_OUT

PC BOARD LAYOUT

Refer to application note AN-1112 for PC board guidelines for the DSBGA package.

The LM34917A 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 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.

The power dissipation within the LM34917A can be approximated by determining the total conversion loss (P_IN

P_OUT), and then subtracting the power losses in the free-wheeling diode and the inductor. The power loss in the

diode is approximately:

P_D1= Iout x V_Fx (1-D)

(20)

where Iout is the load current, V_Fis the diode’s forward voltage drop, and D is the on-time duty cycle. The power

loss in the inductor is approximately:

P_L1= Iout²x R_Lx 1.1

(21)

where R_Lis 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 LM34917A will produce excessive junction temperatures during

normal operation, good use of the PC board’s ground plane can help to dissipate heat. Additionally 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.

LM34917A

www.ti.com.cn

ZHCS593D –DECEMBER 2007–REVISED MARCH 2013

REVISION HISTORY

Changes from Revision C (March 2013) to Revision D

Page

•

Changed layout of National Data Sheet to TI format .......................................................................................................... 16

PACKAGE OPTION ADDENDUM

www.ti.com

10-Dec-2020

PACKAGING INFORMATION

Orderable Device

Status Package Type Package Pins Package

Eco Plan

Lead finish/

Ball material

MSL Peak Temp

Op Temp (°C)

Device Marking

Samples

Drawing

Qty

(1)

(2)

(3)

(4/5)

(6)

LM34917ATL/NOPB

ACTIVE

DSBGA

YZR

250

RoHS & Green

SNAGCU

Level-1-260C-UNLIM

-40 to 125

SRHA

⁽¹⁾The marketing status values are defined as follows:

ACTIVE: Product device recommended for new designs.

LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.

NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.

PREVIEW: Device has been announced but is not in production. Samples may or may not be available.

OBSOLETE: TI has discontinued the production of the device.

⁽²⁾RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance

do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may

reference these types of products as "Pb-Free".

RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.

Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of <=1000ppm threshold. Antimony trioxide based

flame retardants must also meet the <=1000ppm threshold requirement.

⁽³⁾MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.

⁽⁴⁾There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device.

⁽⁵⁾Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation

of the previous line and the two combined represent the entire Device Marking for that device.

(6)

Lead finish/Ball material - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead finish/Ball material values may wrap to two

lines if the finish value exceeds the maximum column width.

Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information

provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and

continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.

TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.

In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.

Addendum-Page 1

PACKAGE MATERIALS INFORMATION

www.ti.com

9-Aug-2022

TAPE AND REEL INFORMATION

REEL DIMENSIONS

TAPE DIMENSIONS

Reel

Diameter

Cavity

A0 Dimension designed to accommodate the component width

B0 Dimension designed to accommodate the component length

K0 Dimension designed to accommodate the component thickness

Overall width of the carrier tape

P1 Pitch between successive cavity centers

Reel Width (W1)

QUADRANT ASSIGNMENTS FOR PIN 1 ORIENTATION IN TAPE

Sprocket Holes

Q1 Q2

Q3 Q4

Q1 Q2

Q3 Q4

User Direction of Feed

Pocket Quadrants

*All dimensions are nominal

Device

Package Package Pins

Type Drawing

SPQ

Reel

Pin1

Diameter Width (mm) (mm) (mm) (mm) (mm) Quadrant

(mm) W1 (mm)

LM34917ATL/NOPB

DSBGA

YZR

250

178.0

8.4

2.01

2.57

0.76

4.0

8.0

Pack Materials-Page 1

PACKAGE MATERIALS INFORMATION

www.ti.com

9-Aug-2022

TAPE AND REEL BOX DIMENSIONS

Width (mm)

*All dimensions are nominal

Device

Package Type Package Drawing Pins

DSBGA YZR 12

SPQ

Length (mm) Width (mm) Height (mm)

208.0 191.0 35.0

LM34917ATL/NOPB

250

Pack Materials-Page 2

MECHANICAL DATA

YZR0012xxx

0.600±0.075

TLA12XXX (Rev C)

D: Max = 2.33 mm, Min =2.269 mm

E: Max = 1.987 mm, Min =1.926 mm

4215049/A

12/12

A. All linear dimensions are in millimeters. Dimensioning and tolerancing per ASME Y14.5M-1994.

B. This drawing is subject to change without notice.

NOTES:

www.ti.com

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LM34917ATL/NOPB [TI]

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