LT3837EFE-PBF LINER [Linear Technology], LT3837EFE-PBF Datasheet

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LT3837EFE-PBF

Manufacturer Part Number
LT3837EFE-PBF
Description
Isolated No-Opto Synchronous Flyback Controller
Manufacturer
LINER [Linear Technology]
Datasheet
FEATURES
APPLICATIONS
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TYPICAL APPLICATION
All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 6498466, 5841643.
2N3906
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Senses Output Voltage Directly from Primary Side
Winding—No Optoisolator Required
Synchronous Driver for High Effi ciency
Supply Voltage Range 4.5V to 20V
Accurate Regulation Without User Trims
Programmable Switching Frequency from
50kHz to 250kHz
Synchronizable
Load Compensation
Undervoltage Lockout
Available in a Thermally Enhanced 16-Lead
TSSOP Package
Isolated Medium Power (10W to 60W) Supplies
Instrumentation Power Supplies
Isolated Medical Supplies
22.1k
3k
20k
BAS16
22μF
3.3nF
1.37k
150k
1nF
9V –18V to 3.3V at 10A Isolated Converter
100k
47pF
12k
0.1μF
FB
V
OSC
SFST
t
ENDLY
PGDLY
R
C
ON
C
CMP
CMP
LT3837
GND
V
CC
SENSE
SENSE
UVLO
PG
SG
+
330Ω
86.6k
15k
Si7852DP
8mΩ
0.1μF
10Ω
2.2nF
Si7336ADP
10μF
2.2nF
V
IN
Synchronous Flyback Controller
B0540W
10k
47μF 3
10Ω
FMMT618
FMMT718
100nH
BAT54
15Ω
OUTPUT 3.3V/10A
DESCRIPTION
The LT
designed for medium power fl yback topologies. A typical
application is 10W to 60W with the part powered from a
DC supply.
The LT3837 is a current mode controller that regulates an
output voltage based on sensing the secondary voltage
via a transformer winding during fl yback. This allows for
tight output regulation without the use of an optoisolator,
improving dynamic response and reliability. Synchronous
rectifi cation increases converter effi ciency and improves
output cross regulation in multiple output converters.
The LT3837 operates in forced continuous conduction
mode which improves cross regulation in multiple winding
applications. Switching frequency is user programmable
and can be externally synchronized. The part also has load
compensation, undervoltage lockout and soft-start circuitry.
The LT3837 is available in a thermally enhanced 16-pin
TSSOP package.
220μF
3837 TA01
1μF
®
3837 is an isolated switching regulator controller
Isolated No-Opto
3.60
3.50
3.40
3.30
3.20
3.10
3.00
90
86
82
78
76
88
84
80
Regulation vs Load Current
2
2
Effi ciency vs Load Current
3
3
4
4
9V
LOAD CURRENT (A)
LOAD CURRENT (A)
IN
5
5
18V
9V
6
6
IN
IN
7
7
8
8
18V
LT3837
IN
3837 TA01b
3837 TA01c
9
9
10
10
3837fa
1

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LT3837EFE-PBF Summary of contents

Page 1

FEATURES n Senses Output Voltage Directly from Primary Side Winding—No Optoisolator Required n Synchronous Driver for High Effi ciency n Supply Voltage Range 4.5V to 20V n Accurate Regulation Without User Trims n Programmable Switching Frequency from 50kHz to 250kHz ...

Page 2

... LT3837EFE#TRPBF LEAD BASED FINISH TAPE AND REEL LLT3837EFE LT3837EFE#TR Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. For more information on lead free part marking, go to: For more information on tape and reel specifi cations, go to: ELECTRICAL CHARACTERISTICS temperature range, otherwise specifi ...

Page 3

ELECTRICAL CHARACTERISTICS temperature range, otherwise specifi cations are 27.4k 90k, unless otherwise specifi ed. PGDLY ENDLY PARAMETER Feedback Amplifi er Voltage Gain Soft-Start Charging Current Soft-Start Discharge Current Control Pin Threshold ( Gate ...

Page 4

LT3837 TYPICAL PERFORMANCE CHARACTERISTICS V Shutdown Current CC vs Temperature UVLO 14V –50 – 100 TEMPERATURE (°C) 3837 G02 SENSE ...

Page 5

TYPICAL PERFORMANCE CHARACTERISTICS Feedback Amplifi er Source and Sink Current vs Temperature 70 SOURCE CURRENT SINK CURRENT 1. –50 – 100 TEMPERATURE (°C) ...

Page 6

LT3837 TYPICAL PERFORMANCE CHARACTERISTICS PG Delay Time vs Temperature 300 250 R = 27.4k PGDLY 200 150 R = 16.7k PGDLY 100 50 0 –50 –30 – TEMPERATURE (°C) PIN FUNCTIONS SG (Pin 1): Synchronous gate drive ...

Page 7

PIN FUNCTIONS UVLO (Pin 10): A resistive divider from V an undervoltage lockout based upon V When the UVLO pin is below its threshold, the gate drives are disabled, but the part draws its normal quiescent current from V . ...

Page 8

LT3837 BLOCK DIAGRAM 1.25V REFERENCE ( DISABLE + 0.8V – UVLO 10 OSC 7 OSCILLATOR SYNC PGDLY 15 ENDLY 4 8 CLAMPS 0.7 + 1.3 – 1.25V 3V INTERNAL REGULATOR S ...

Page 9

FLYBACK FEEDBACK AMPLIFIER LT3837 FEEDBACK AMP 1.25V – + COLLAPSE DETECT ENABLE TIMING DIAGRAM PRIMARY SIDE MOSFET DRAIN VOLTAGE PG VOLTAGE SG VOLTAGE V FLBK – ...

Page 10

LT3837 OPERATION The LT3837 is a current mode switcher controller IC designed specifi cally for use in an isolated fl yback topology employing synchronous rectifi cation. The LT3837 operation is similar to traditional current mode switchers. The major difference is ...

Page 11

OPERATION Minimum Output Switch On-Time (t The LT3837 affects output voltage regulation via fl yback pulse action. If the output switch is not turned on, there is no fl yback pulse and output voltage information is not available. This causes ...

Page 12

LT3837 OPERATION LOAD COMP – CMP CMP Figure 1. Load Compensation Diagram Figure 1 shows the block diagram of the load compensa- tion ...

Page 13

APPLICATIONS INFORMATION Primary Winding Feedback The previous work was developed using a separate wind- ing for voltage feedback possible to use the primary winding as the feedback winding as well. This can simplify the design of the transformer. ...

Page 14

LT3837 APPLICATIONS INFORMATION Transformer Design Transformer design/specifi cation is the most critical part of a successful application of the LT3837. The following sections provide basic information about designing the transformer and potential tradeoffs. If you need help, the LTC Applications ...

Page 15

APPLICATIONS INFORMATION It then reverts to a potentially stable state whereby the top of the leakage spike is the control point, and the trailing edge of the leakage spike triggers the collapse detect circuitry. This typically reduces the output voltage ...

Page 16

LT3837 APPLICATIONS INFORMATION Optimization might show that a more effi cient solution is obtained at higher peak current but lower inductance and the associated winding series resistance. A simple spreadsheet program is useful for looking at tradeoffs. Transformer Core Selection ...

Page 17

APPLICATIONS INFORMATION Size R using worst-case conditions, minimum L SENSE V and maximum V . Continuing the example, let us SENSE IN assume that our worst-case conditions yield an I above nominal 10.41A . If there is ...

Page 18

LT3837 APPLICATIONS INFORMATION with slope compensation and system stability. Keep the sync pulse width greater than 500ns. Selecting Timing Resistors There are three internal “one-shot” times that are pro- grammed by external application resistors: minimum on-time, enable delay time and ...

Page 19

APPLICATIONS INFORMATION Primary Gate Delay Time (PGDLY) Primary gate delay is the programmable time from the turn-off of the synchronous MOSFET to the turn-on of the primary side MOSFET. Correct setting eliminates overlap between the primary side switch and secondary ...

Page 20

LT3837 APPLICATIONS INFORMATION If we wanted a V -referred trip point of 8.4V, with 0. hysteresis (on at 8.4V, off at 8.1V use ...

Page 21

APPLICATIONS INFORMATION Short-Circuit Conditions Loss of current limit is possible under certain conditions such as an output short circuit. If the duty cycle exhib- ited by the minimum on-time is greater than the ratio of secondary winding voltage (referred-to-primary) divided ...

Page 22

LT3837 APPLICATIONS INFORMATION where N refl ects the turns ratio of that secondary-to-pri- SP mary winding the primary-side leakage inductance LKG and C is the primary-side capacitance (mostly from the the primary-side power MOSFET). A ...

Page 23

APPLICATIONS INFORMATION The parasitic inductance creates an LC tank with the MOSFET gate capacitance. In less than ideal layouts, a series resistance of 5Ω or more may help to dampen the ringing at the expense of slightly slower rise and ...

Page 24

LT3837 APPLICATIONS INFORMATION The other 1% is due to the bulk C component, so use: I ≥ OUT C OUT 1% • V • f OUT OSC In many applications the output capacitor is created from multiple capacitors to achieve ...

Page 25

APPLICATIONS INFORMATION PC Board Layout Considerations In order to minimize switching noise and improve output load regulation, connect the GND pin of the LT3837 directly to the ground terminal of the V CC the bottom terminal of the current sense ...

Page 26

LT3837 TYPICAL APPLICATION 36V 10k MMBT3904 10μF 0.1μF 7.5V 100k 1% 29. UVLO PGDLY 3.01k 17.4k t SYNC 12k 100k PIN 10/11/12 TO 7/8/ 7MIL C ...

Page 27

PACKAGE DESCRIPTION 6.60 ±0.10 4.50 ±0.10 SEE NOTE 4 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) (.020 – .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS 2. DIMENSIONS ARE IN 3. DRAWING NOT ...

Page 28

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