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LT1375/LT1376

13756fd

APPLICATIO S

DESCRIPTIO

FEATURES

TYPICAL APPLICATIO

1.5A, 500kHz Step-Down

Switching Regulators

■

Constant 500kHz Switching Frequency

■

Uses All Surface Mount Components

■

Inductor Size Reduced to 5

■

Easily Synchronizable

■

Saturating Switch Design: 0.4Ω

■

Effective Supply Current: 2.5mA

■

Shutdown Current: 20µA

■

Cycle-by-Cycle Current Limiting

■

Portable Computers

■

Battery-Powered Systems

■

Battery Charger

■

Distributed Power

The LT

1375/LT1376 are 500kHz monolithic buck mode

switching regulators. A 1.5A switch is included on the die

along with all the necessary oscillator, control and logic

circuitry. High switching frequency allows a considerable

reduction in the size of external components. The topology

is current mode for fast transient response and good loop

stability. Both fixed output voltage and adjustable parts are

available.

A special high speed bipolar process and new design

techniques achieve high efficiency at high switching fre-

quency. Efficiency is maintained over a wide output cur-

rent range by using the output to bias the circuitry

and by

utilizing a supply boost

capacitor to saturate the power

switch. A shutdown signal will reduce supply current to

20µA on both parts. The LT1375 can be externally syn-

chronized from 580kHz to 900kHz with logic level inputs.

The LT1375/LT1376 fit into standard 8-pin PDIP and SO

packages, as well as a fused lead 16-pin SO with much

lower thermal resistance. Full cycle-by-cycle short-cir-

cuit protection and thermal shutdown are provided.

Standard surface mount external parts are used, includ-

ing the inductor and capacitors.

For low input voltage applications with 3.3V output, see

LT1507. This is a functionally identical part that can

operate with input voltages between 4.5V and 12V.

5V Buck Converter

LOAD CURRENT (A)

EFFICIENCY (%)

100

1.00

1375/76 TA02

0.25

0.50

0.75

1.25

OUT

= 5V

= 10V

L = 10µH

Efficiency vs Load Current

BOOST

LT1376-5

OUTPUT**

5V, 1.25A

* RIPPLE CURRENT ≥ I

OUT

** INCREASE L1 TO 10µH FOR LOAD CURRENTS ABOVE 0.6A AND TO 20µH ABOVE 1A

†

FOR INPUT VOLTAGE BELOW 7.5V, SOME RESTRICTIONS MAY APPLY.

SEE APPLICATIONS INFORMATION.

INPUT

†

TO 25V

1375/76 TA01

0.1µF

3.3nF

1N5818

100µF, 10V

SOLID

TANTALUM

C3*

10µF TO

50µF

1N914

L1**

5µH

BIAS

GND

DEFAULT

= ON

SHDN

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

Summary of content (28 pages)

PAGE 1
LT1375/LT1376 1.5A, 500kHz Step-Down Switching Regulators FEATURES Constant 500kHz Switching Frequency Uses All Surface Mount Components Inductor Size Reduced to 5µH Easily Synchronizable Saturating Switch Design: 0.4Ω Effective Supply Current: 2.5mA Shutdown Current: 20µA Cycle-by-Cycle Current Limiting ■ ■ ■ ■ ■ ■ ■ ■ is current mode for fast transient response and good loop stability. Both fixed output voltage and adjustable parts are available.
PAGE 2
LT1375/LT1376 U W W W ABSOLUTE MAXIMUM RATINGS (Note 1) Input Voltage LT1375/LT1376 .................................................. 25V LT1375HV/LT1376HV ........................................ 30V BOOST Pin Voltage LT1375/LT1376 .................................................. 35V LT1375HV/LT1376HV ........................................ 40V SHDN Pin Voltage ..................................................... 7V BIAS Pin Voltage ......................................................
PAGE 3
LT1375/LT1376 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. TJ = 25°C, VIN = 15V, VC = 1.5V, boost open, switch open, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS 2.39 2.36 2.42 ● 2.45 2.48 V V 4.94 4.90 5.0 ● 5.06 5.10 V V 7 10 14 kΩ 0.01 0.01 0.03 0.03 %/ V %/V 0.5 1.
PAGE 4
LT1375/LT1376 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. TJ = 25°C, VIN = 15V, VC = 1.5V, boost open, switch open, unless otherwise noted. PARAMETER CONDITIONS Shutdown Thresholds VC Open Device Shutting Down Device Starting Up VC Open LT1375HV/LT1376HV Device Shutting Down LT1375HV/LT1376HV Device Starting Up Minimum Synchronizing Amplitude (LT1375 Only) MIN TYP MAX UNITS ● ● 0.15 0.
PAGE 5
LT1375/LT1376 U W TYPICAL PERFORMANCE CHARACTERISTICS Standby and Shutdown Thresholds Shutdown Pin Bias Current CURRENT REQUIRED TO FORCE SHUTDOWN (FLOWS OUT OF PIN). AFTER SHUTDOWN, CURRENT DROPS TO A FEW µA VSHUTDOWN = 0V 300 200 8 AT 2.38V STANDBY THRESHOLD (CURRENT FLOWS OUT OF PIN) 4 INPUT SUPPLY CURRENT (µA) STANDBY SHUTDOWN PIN VOLTAGE (V) CURRENT (µA) 400 2.36 2.32 0.8 START-UP 0.
PAGE 6
LT1375/LT1376 U W TYPICAL PERFORMANCE CHARACTERISTICS Maximum Load Current at VOUT = 5V Maximum Load Current at VOUT = 3.3V Maximum Load Current at VOUT = 10V 1.50 1.50 1.50 L = 20µH VOUT = 10V L = 20µH 1.25 1.25 L = 10µH 1.00 L = 5µH L = 20µH 1.25 L = 10µH 0.75 L = 5µH 0.50 CURRENT (A) 1.00 CURRENT (A) CURRENT (A) L = 10µH 0.75 L = 5µH 0.75 0.50 0.50 0.25 1.00 0.25 0.25 VOUT = 5V VOUT = 3.
PAGE 7
LT1375/LT1376 U U U PIN FUNCTIONS VIN: This is the collector of the on-chip power NPN switch. This pin powers the internal circuitry and internal regulator when the BIAS pin is not present. At NPN switch on and off, high dl/dt edges occur on this pin. Keep the external bypass and catch diode close to this pin. All trace inductance on this path will create a voltage spike at switch off, adding to the VCE voltage across the internal NPN.
PAGE 8
LT1375/LT1376 W BLOCK DIAGRAM connected to an external voltage higher than 3V, bias power will be drawn from the external source (typically the regulated output voltage). This will improve efficiency if the BIAS pin voltage is lower than regulator input voltage. High switch efficiency is attained by using the BOOST pin to provide a voltage to the switch driver which is higher than the input voltage, allowing switch to be saturated. This boosted voltage is generated with an external capacitor and diode.
PAGE 9
LT1375/LT1376 U U W U APPLICATIONS INFORMATION LT1375/LT1376 VSW TO FREQUENCY SHIFTING 1.6V ERROR AMPLIFIER + R5 5k OUTPUT 5V Q1 2.4V – R3 1k R1 R4 1k FB Q2 VC + R2 5k GND 1375/76 F02 Figure 2. Frequency and Current Limit Foldback Please read the following if divider resistors are increased above the suggested values. R1 = ( ) R2 VOUT − 2.42 2.42 Table 1 OUTPUT VOLTAGE (V) R2 (kΩ) R1 (NEAREST 1%) (kΩ) % ERROR AT OUTPUT DUE TO DISCREET 1% RESISTOR STEPS 3 4.99 1.21 + 0.
PAGE 10
LT1375/LT1376 U W U U APPLICATIONS INFORMATION pin when output voltage is low. The equivalent circuitry is shown in Figure 2. Q1 is completely off during normal operation. If the FB pin falls below 1V, Q1 begins to conduct current and reduces frequency at the rate of approximately 5kHz/µA. To ensure adequate frequency foldback (under worst-case short-circuit conditions), the external divider Thevinin resistance must be low enough to pull 150µA out of the FB pin with 0.6V on the pin (RDIV ≤ 4k).
PAGE 11
LT1375/LT1376 U U W U APPLICATIONS INFORMATION Example: with L = 2µH, VOUT = 5V, and VIN(MAX) = 15V, 1.5) ⎛⎝ 500 • 10 ⎞⎠ ⎛⎝ 2 • 10 ⎞⎠ (15) ( = 338mA )= 2(5)(15 − 5) 2 IOUT (MAX 3 −6 The main reason for using such a tiny inductor is that it is physically very small, but keep in mind that peak-to-peak inductor current will be very high. This will increase output ripple voltage. If the output capacitor has to be made larger to reduce ripple voltage, the overall circuit could actually wind up larger.
PAGE 12
LT1375/LT1376 U W U U APPLICATIONS INFORMATION 5. After making an initial choice, consider the secondary things like output voltage ripple, second sourcing, etc. Use the experts in the Linear Technology’s applications department if you feel uncertain about the final choice. They have experience with a wide range of inductor types and can tell you about the latest developments in low profile, surface mounting, etc. Table 2 VENDOR/ PART NO.
PAGE 13
LT1375/LT1376 U U W U APPLICATIONS INFORMATION discharge surges, such as when the regulator output is dead shorted, do not harm the capacitors. Unlike the input capacitor, RMS ripple current in the output capacitor is normally low enough that ripple current rating is not an issue. The current waveform is triangular with a typical value of 200mARMS.
PAGE 14
LT1375/LT1376 U W U U APPLICATIONS INFORMATION regulator input voltage. Average forward current in normal operation can be calculated from: ID(AVG) = ( IOUT VIN − VOUT ) VIN This formula will not yield values higher than 1A with maximum load current of 1.25A unless the ratio of input to output voltage exceeds 5:1. The only reason to consider a larger diode is the worst-case condition of a high input voltage and overloaded (not shorted) output.
PAGE 15
LT1375/LT1376 U U W U APPLICATIONS INFORMATION RFB LT1375/LT1376 2.38V IN INPUT OUTPUT VSW + STANDBY RHI – 3.5µA + SHDN + TOTAL SHUTDOWN RLO C1 0.37V – GND 1375/76 F04 Figure 4. Undervoltage Lockout or latch low under low source voltage conditions. UVLO prevents the regulator from operating at source voltages where these problems might occur. Threshold voltage for lockout is about 2.38V, slightly less than the internal 2.42V reference voltage. A 3.
PAGE 16
LT1375/LT1376 U W U U APPLICATIONS INFORMATION (magnetic) radiation is minimized by keeping catch diode, switch pin, and input bypass capacitor leads as short as possible. E field radiation is kept low by minimizing the length and area of all traces connected to the switch pin and BOOST pin. A ground plane should always be used under the switcher circuitry to prevent interplane coupling. A suggested layout for the critical components is shown in Figure 5.
PAGE 17
LT1375/LT1376 U W U U APPLICATIONS INFORMATION PARASITIC RESONANCE Resonance or “ringing” may sometimes be seen on the switch node (see Figure 7). Very high frequency ringing following switch rise time is caused by switch/diode/input capacitor lead inductance and diode capacitance. Schottky diodes have very high “Q” junction capacitance that can ring for many cycles when excited at high frequency.
PAGE 18
LT1375/LT1376 U W U U APPLICATIONS INFORMATION practice therefore to simply use the worst-case value and assume that RMS ripple current is one half of load current. At maximum output current of 1.5A for the LT1376, the input bypass capacitor should be rated at 0.75A ripple current. Note however, that there are many secondary considerations in choosing the final ripple current rating.
PAGE 19
LT1375/LT1376 U U W U APPLICATIONS INFORMATION 8.0 (A) MINIMUM VOLTAGE TO START WITH STANDARD CIRCUIT INPUT VOLTAGE (V) 7.5 (B) (A) (B) MINIMUM VOLTAGE TO RUN WITH STANDARD CIRCUIT 7.0 (C) (C) MINIMUM VOLTAGE TO START WITH PNP 6.5 6.0 (D) (D) MINIMUM VOLTAGE TO RUN WITH PNP 5.5 5.0 0.001 0.01 0.1 LOAD CURRENT (A) 1 1375/76 F09 Figure 9. Minimum Input Voltage D1 1N914 C2 0.
PAGE 20
LT1375/LT1376 U U W U APPLICATIONS INFORMATION introduce multiple poles into the feedback loop. The inductor and output capacitor on a conventional stepdown converter actually form a resonant tank circuit that can exhibit peaking and a rapid 180° phase shift at the resonant frequency. By contrast, the LT1376 uses a “current mode” architecture to help alleviate phase shift created by the inductor. The basic connections are shown in Figure 12.
PAGE 21
LT1375/LT1376 U U W U APPLICATIONS INFORMATION 77dB at low frequency, rolling off to unity-gain at 20kHz. Phase shows a two-pole characteristic until the ESR of the output capacitor brings it back above 10kHz. Phase margin is about 60° at unity-gain. Analog experts will note that around 1kHz, phase dips very close to the zero phase margin line. This is typical of switching regulators, especially those that operate over a wide range of loads.
PAGE 22
LT1375/LT1376 U W U U APPLICATIONS INFORMATION How Do I Test Loop Stability? The “standard” compensation for LT1376 is a 3.3nF capacitor for CC, with RC = 0. While this compensation will work for most applications, the “optimum” value for loop compensation components depends, to various extent, on parameters which are not well controlled.
PAGE 23
LT1375/LT1376 U U W U APPLICATIONS INFORMATION Keep in mind that this procedure does not take initial component tolerance into account. You should see fairly clean response under all load and line conditions to ensure that component variations will not cause problems. One note here: according to Murphy, the component most likely to be changed in production is the output capacitor, because that is the component most likely to have manufacturer variations (in ESR) large enough to cause problems.
PAGE 24
LT1375/LT1376 U W U U APPLICATIONS INFORMATION POSITIVE-TO-NEGATIVE CONVERTER Maximum load current: The circuit in Figure 18 is a classic positive-to-negative topology using a grounded inductor. It differs from the standard approach in the way the IC chip derives its feedback signal, however, because the LT1376 accepts only positive feedback signals, the ground pin must be tied to the regulated negative output.
PAGE 25
LT1375/LT1376 U U W U APPLICATIONS INFORMATION INDUCTOR VALUE Minimum inductor discontinuous mode: Unlike buck converters, positive-to-negative converters cannot use large inductor values to reduce output ripple voltage. At 500kHz, values larger than 25µH make almost no change in output ripple. The graph in Figure 19 shows peak-to-peak output ripple voltage for a 5V to – 5V converter versus inductor value.
PAGE 26
LT1375/LT1376 U U W U APPLICATIONS INFORMATION surface mount solid tantalum capacitor, but the final capacitor chosen must be looked at carefully for ESR characteristics. Peak diode current: Continuous Mode = ) + (V )(V ) V 2(L)(f)(V + V ) 2(I )(V ) Discontinuous Mode = (L)(f) Ripple Current in the Input and Output Capacitors IOUT ( )( ) IN + VOUT IN IN Positive-to-negative converters have high ripple current in both the input and output capacitors.
PAGE 27
LT1375/LT1376 U PACKAGE DESCRIPTION N8 Package 8-Lead PDIP (Narrow 0.300) (LTC DWG # 05-08-1510) .400* (10.160) MAX 8 7 6 5 1 2 3 4 .255 ± .015* (6.477 ± 0.381) .300 – .325 (7.620 – 8.255) .065 (1.651) TYP .008 – .015 (0.203 – 0.381) ( +.035 .325 –.015 8.255 +0.889 –0.381 .130 ± .005 (3.302 ± 0.127) .045 – .065 (1.143 – 1.651) ) .120 (3.048) .020 MIN (0.508) MIN .018 ± .003 .100 (2.54) BSC (0.457 ± 0.076) N8 1002 NOTE: 1.
PAGE 28
LT1375/LT1376 U PACKAGE DESCRIPTION S Package 16-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) .386 – .394 (9.804 – 10.008) NOTE 3 .045 ±.005 .050 BSC 16 N 15 14 13 12 11 10 9 N .245 MIN .160 ±.005 .150 – .157 (3.810 – 3.988) NOTE 3 .228 – .244 (5.791 – 6.197) 1 .030 ±.005 TYP 2 3 N/2 N/2 RECOMMENDED SOLDER PAD LAYOUT 1 .010 – .020 × 45° (0.254 – 0.508) 2 3 4 5 6 7 8 .053 – .069 (1.346 – 1.752) .008 – .010 (0.203 – 0.254) .004 – .010 (0.101 – 0.