HT7937 -- PFM Step-up DC/DC Converter

HT7937
Built-in OVP White LED Step-up Converter
Features
· Integrated open-circuit protection
· Over voltage protection
· Low standby current: 0.1mA (typ.) (VSHDN=0V)
· Matches LED current
· Up to 84% efficiency at VIN=3V, 3LEDs,
· Small value inductor and capacitors
ILED= 20mA
· Small outline SOT23-6 package
· 1.2MHz fixed switching frequency
Applications
· Cellular phones
· Handheld devices
· PDAs
· White LED display backlighting
· DSCs
General Description
The HT7937 is high efficiency boost converter with a
constant current output to provide back light functions in
handheld devices. Applications with series connected
LEDs ensure constant and identical LED currents resulting in uniform brightness. A continuous LED output
current is setup using the FB pin regulated voltage
across an external sense resistor, RFB connected between the FB pin and ground. The integrated open load
protection circuitry prevents damage resulting from an
open circuit condition. The low 95mV feedback voltage
minimises power losses in the current setting resistor
which improves efficiency. The HT7937 has a high
switching frequency of up to 1.2MHz which permits the
use of lower value extremely small outline inductor and
filter capacitor.
The HT7937 is supplied in a space-saving, 6-lead
SOT23-6 package type.
Selection Table
Note:
Part No.
Package
Marking
HT7937
SOT23-6
7937#
7937+
Both lead free and green compound devices are available.
²#² stands for Lead-free devices.
²+² stands for green compound devices, which are Lead-free and Halogen-free.
Rev 1.30
1
November 9, 2010
HT7937
Block Diagram
O V P
2 8 V
S W
S H D N
C o n tro l
L o g ic
F B
9 5 m V
å
G N D
1 .2 M H z
O s c illa to r
V IN
Pin Assignment
S O T 2 3 -6
V IN
6
O V P
5
S H D N
4
T o p V ie w
1
2
3
S W
G N D
F B
Pin Description
Pin No.
Pin Name
Description
1
SW
2
GND
3
FB
4
SHDN
5
OVP
For over voltage protection connect to the output.
6
VIN
Input voltage pin. 2.5V to 5.5V for internal circuitry.
Rev 1.30
Switching pin
Ground pin
Output voltage sense node. Connect the cathode of the LED to this pin. A resistor from this
pin to ground sets the LED current. Internally compares to 95mV (Typ.).
Shutdown device pin. Connect to 1.5V or higher to enable device (ON), 0.3V or lower to
disable device (OFF).
2
November 9, 2010
HT7937
Absolute Maximum Ratings
Input Voltage..............................................................6V
SW Voltage..............................................................36V
FB Voltage .................................................................6V
SHDN ........................................................................6V
OVP Voltage ............................................................36V
Operating Temperature Range ...............-40°C to 85°C
Maximum Junction Temperature..........................125°C
Note: These are stress ratings only. Stresses exceeding the range specified under ²Absolute Maximum Ratings² may
cause substantial damage to the device. Functional operation of this device at other conditions beyond those listed
in the specification is not implied and prolonged exposure to extreme conditions may affect device reliability.
Electrical Characteristics
Symbol
VIN
Parameter
VSHDN=3V; VIN=3V; Ta=25°C (Unless otherwise specified)(Note)
Test Conditions
Min.
Typ.
Max.
Unit
¾
2.5
3
5.5
V
Switching
¾
1.0
1.25
mA
Non-switching
¾
50
100
mA
VSHDN=0V
¾
0.1
1
mA
3 LEDs
85
95
105
mV
0.8
1.2
1.6
MHz
85
90
¾
%
Input Voltage
IIN
Supply Current
VFB
Feedback Voltage
fOSC
Switching Frequency
DC
Maximum Duty Cycle
RDS(ON)
SW On Resistance
¾
¾
1.4
5
W
ISW(OFF)
Switch Leakage Current
¾
¾
0.1
1
mA
VIH
SHDN Voltage High
¾
1.5
¾
¾
V
VIL
SHDN Voltage Low
¾
¾
¾
0.3
V
VOVP
OVP Threshold
23
28
33
V
Note:
Measurement at SW pin
No load
Specifications are production tested at Ta=25 degree. Specifications over -40 to 85 degree operating
temperature range are assured by design, characterization.
Rev 1.30
3
November 9, 2010
HT7937
Functional Description
· Shutdown
¨
The shutdown pin, SHDN, must not be allowed to
float. When the SHDN pin voltage is taken below 0.3V,
the internal MOSFET, voltage reference, error amplifier, comparators and biasing circuitry will all be
switched off reducing the quiescent supply current to
less than 1mA. If the SHDN pin has a value greater
than 1.5V, then the device will be fully enabled and operational. This pin also can be used as a PWM signal
from 100Hz to 1kHz to allow brightness control.
A DC signal on the FB pin
This method of dimming control uses a DC voltage
circuit as shown in Figure 2.
The LED brightness is directly proportional to the
LED current which is given by the following equation:
Where
VFB = Feedback voltage is 95mV
VDC = DC voltage
· Over voltage protection - OVP
R1 and R2 >> RFB
With an open circuit output, such as when no LEDs
are connected, the FB pin will be pulled down to
ground via the sense resistor RFB. As the device will
now react by trying to increase the output voltage by
generating a maximum duty cycle signal, this may
cause the SW pin to exceed its maximum rated voltage, which may damage the internal N-MOS switching transistor.
The OVP function is designed to prevent damage to
the internal NMOS switching transistor. When the output voltage rises above the OVP threshold voltage,
typically 28V, the converter will clamp the output voltage to this level. When the output voltage returns to a
value below the OVP threshold, it will automatically resume normal switching operation.
C 1
1 m F
¨
P W M
f= 1 0 0 H z ~ 1 k H z
V IN
H T 7 9 3 7
V
U p to
6 W L E D s
F B
R 1
1 k W
D C
R
R 2
5 1 k W
F B
4 .7 W
Where
VFB = Feedback voltage is 95mV
VPWM = PWM high level voltage
D = PWM duty cycle
R1 and R2 >> RFB
O V P
G N D
V
F B
The PWM control circuitry is connected to the FB
pin. Reducing the duty cycle on the PWM signal results in increased LEDs brightness levels. The LED
brightness is directly proportional to the LED current which is given by the following equation:
C 2
1 m F
1 N 5 8 1 9
S W
S H D N
O V P
A filtered PWM signal on the FB pin
For frequencies greater than 1kHz, dimming can be
implemented by using the circuit shown in Figure 3.
D 1
1 0 m H
C 2
1 m F
1 N 5 8 1 9
S W
V IN
Figure 2. Dimming Control Using a DC Voltage
The magnitude of the PWM signal should be higher
than the enable voltage of the SHDN pin, the LEDs
operate with either zero or full current. The average
LED current is proportional to the duty cycle of the
applied PWM signal with a duty cycle increase resulting in higher LEDs brightness. Typical PWM frequencies should be between 100Hz and 1kHz.
IN
1 0 m H
H T 7 9 3 7
A PWM signal on the SHDN pin
A PWM signal is applied to the SHDN pin as shown
in Figure 1.
L
C 1
1 m F
G N D
There are three methods to control the LEDs brightness as listed below:
V
IN
S H D N
· Dimming control
¨
D 1
L
V
PWM frequency >>
U p to
6 W L E D s
F B
R
D 1
L
V
F B
4 .7 W
IN
C 1
1 m F
1 0 m H
S H D N
O V P
G N D
Figure 1. Dimming Control with PWM Signal
C 2
1 m F
1 N 5 8 1 9
S W
V IN
H T 7 9 3 7
V P W M
0 V
V
F B
F B
R
R 2
5 1 k W
R 3
5 .1 k W
U p to
6 W L E D s
R 1
1 k W
F B
4 .7 W
C 3
0 .1 m F
Figure 3. Dimming Control Using a Filter PWM Signal
Rev 1.30
4
November 9, 2010
HT7937
Component Selection
Where
IL(PEAK) = Peak Inductor Current
CO(ESR) = Output Capacitor¢s Equivalent Series
Resistance
IO = Output Current
VO = Output Voltage
Vi = Input Voltage
CO = Output Capacitance
FS = Switching Frequency is 1.2MHz
· Setting the LED Current
The step-up converter regulates the LED current by
regulating the voltage across the current sense resistor, RFB. To ensure the generation of accurate LED
currents, it is recommended that a precision resistor is
used for RFB. The voltage across the sense resistor is
regulated to the internal reference voltage of 95mV.
The LED current is calculated using the following
equation:
VFB
ILED =
RFB
Where
VFB = Feedback voltage is 95mV
Layout Considerations
Circuit board layout is a very important consideration for
switching regulators if they are to function properly. Poor
circuit layout may result in related noise problems.
In order to minimise EMI and switching noise, please follow the guidelines below:
· Inductor Selection
The selected inductor must have a saturation current
greater than the maximum peak current of the step-up
converter. A recommended value of inductor for 3 to 6
white LED applications is 4.7mH to 22mH. For good efficiency the inductor should have low core loss and
low DCR (DC Resistance).
The peak inductor current is calculated using the
following equation:
· All tracks should be as wide as possible.
· The input and output capacitors, C1 and C2, should
be placed close to the VIN, VOUT and GND pins.
· The Schottky diode, D1, and inductor, L1, must all be
placed close to the SW pin.
· Feedback resistor, R1, must be placed close to the FB
and GND pins.
· A full ground plane is always helpful for better EMI
performance.
A recommended PCB layout with component locations
is shown below.
Where
VO = Output Voltage
Vi = Input Voltage
IO = Output Current
h = Efficiency
L = Inductance
FS = Switching Frequency is 1.2MHz
· Diode Selection
The diode must have a rating greater than the output
voltage and output current. In switching applications
both forward voltage drop and diode capacitance are
important considerations.
A Schottky diode is used due to its low forward voltage
drop and fast switching speeds which improves efficiency. The Schottky diode has average current of IO,
and a peak current which is the same as the inductor¢s
peak current and a voltage rating at least 1.5 times the
output voltage. A recommended Schottky diode type
is the 1N5819.
Top Layer
· Capacitor Selection
A low ESR (Equivalent Series Resistance) capacitor
should be used for the input/output capacitors to minimize ripples. Both X5R and X7R types are suitable
due to their wider voltage and temperature ranges. Input and output ceramic capacitors of 1mF are recommended for HT7937 applications.
The output ripple voltage is calculated as the following
equation:
Bottom Layer
Rev 1.30
5
November 9, 2010
HT7937
Typical Performance Characteristics
105
90
103
VIN=5V
Reference Voltage (mV)
VIN=4V
85
VIN=3.5V
Efficiency(%)
80
VIN=3V
75
101
99
97
95
3 WLEDs, VIN=3V
L: 10uH, SR0602 10ML(ABC Electronics Corp.)
D1: 1N5819
C1, C2: 1uF
93
91
89
87
70
85
L: 10uH, SR0602 10ML (ABC Electronics Corp.)
D1: 1N5819
C1, C2: 1uF
65
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Input Voltage (V)
Input Voltage VS Reference Voltage
60
0
5
10
15
20
LED Current (mA)
3 WLEDs Efficiency vs. LED Current
90
2.0
Switching Frequency (MHz)
VIN=5V
85
Efficiency(%)
80
VIN=4V
VIN=3.5V
VIN=3V
75
70
3 WLEDs, VIN=3V
L: 10uH, SR0602 10ML(ABC Electronics Corp.)
D1: 1N5819
C1, C2: 1uF
1.6
1.4
1.2
1.0
0.8
L: 10uH, SR0602 10ML (ABC Electronics Corp.)
D1: 1N5819
C1, C2: 1uF
65
1.8
-45
5
10
15
-15
0
15
30
Temperature (
60
0
-30
45
60
75
90
)
Temperature VS Switching Frequency
20
LED Current (mA)
105
95
103
94
Switching Duty Cycle (%)
Reference Voltage (mV)
5 WLEDs Efficiency vs. LED Current
101
99
97
95
3 WLEDs, VIN=3V
L: 10uH, SR0602 10ML(ABC Electronics Corp.)
D1: 1N5819
C1, C2: 1uF
93
91
89
87
93
92
91
90
3 WLEDs, VIN=3V
L: 10uH, SR0602 10ML(ABC Electronics Corp.)
D1: 1N5819
C1, C2: 1uF
89
88
87
86
85
85
-45
-30
-15
0
15
30
Temperature (
45
60
75
90
-45
)
-15
0
15
30
Temperature (
45
60
75
90
)
Temperature VS Switching Duty Cycle
Temperature VS Reference Voltage
Rev 1.30
-30
6
November 9, 2010
HT7937
Start-Up from Shutdown Waveform
Note:
Operation Waveform
Note:
VIN=3.6V, 3 WLEDs, L=10mH, D= 1N5819,
Ci=Co=1mF, ILED=20mA
VIN=3.6V, 3 WLEDs, L=10mH, D=1N5819,
Ci=Co=1mF, ILED=20mA
Start-Up from Shutdown Waveform
Note:
Operation Waveform
Note:
VIN=3.6V, 5 WLEDs, L=10mH, D=1N5819,
Ci=Co=1mF, ILED=20mA
VIN=3.6V, 5 WLEDs, L=10mH, D=1N5819,
Ci=Co=1mF, ILED=20mA
100%
80%
Average ILED / ILED Max
95
Efficiency(%)
90
85
80
6S1P
3S2P
75
70
65
60
VIN=3.6V, L=10uH,
Ci=Co=1uF, 3LEDs
60%
100Hz
200Hz
500Hz
1KHz
40%
20%
2.5
3.0
3.5
4.0
4.5
5.0
VIN(V)
0%
Efficiency vs. VIN
0%
20%
40%
60%
80%
100%
Shutdown PIN PWM Duty
Dimming Control by Shutdown PIN
Rev 1.30
7
November 9, 2010
HT7937
3 WLEDs, VIN=3.6V, f=200Hz, Duty=50%
3 WLEDs, VIN=3.6V, f=1kHz, Duty=50%
3 WLEDs, VIN=3.6V, f=500Hz, Duty=50%
Rev 1.30
8
November 9, 2010
HT7937
Application Circuits
6 WLEDs Application Circuit
V
IN
C 1
1 m F
U p to
6 W L E D s
D 1
L
1 0 m H
C 2
1 m F
1 N 5 8 1 9
V IN
S W
S H D N
O V P
G N D
2 0 m A
F B
R
H T 7 9 3 7
F B
4 .7 W
N o te : "L " S R 0 6 0 2 1 0 0 M L B
( A B C T a iw a n E le c tr o n ic s C o r p .)
Dimming Control with a PWM Signal
IN
C 1
1 m F
P W M
U p to
6 W L E D s
D 1
L
V
1 0 m H
V IN
C 2
1 m F
1 N 5 8 1 9
S W
S H D N
O V P
G N D
F B
H T 7 9 3 7
R
F B
4 .7 W
Dimming Control Using a DC Voltage
IN
C 1
1 m F
U p to
6 W L E D s
D 1
L
V
1 0 m H
V IN
C 2
1 m F
1 N 5 8 1 9
S W
S H D N
O V P
G N D
R 1
1 k W
F B
V D C
0 V ~ 5 V
H T 7 9 3 7
R
F B
R 2
5 1 k W
4 .7 W
Dimming Control Using a Filter PWM Signal
IN
C 1
1 m F
1 0 m H
V IN
C 2
1 m F
1 N 5 8 1 9
S W
S H D N
O V P
G N D
R 1
1 k W
F B
H T 7 9 3 7
R
R 2
5 1 k W
P W M
Rev 1.30
U p to
6 W L E D s
D 1
L
V
R 3
5 .1 k W
9
F B
4 .7 W
C 3
0 .1 m F
November 9, 2010
HT7937
Note:
As the above application circuits are unable to show the full drive capabilities and series/parallel drive combinations of this device the following supplemental information is provided.
For the general full voltage range situation:
1. Vin Variable Voltage of 2.6V~5.5V
2. LED forward voltage of 3.5V max.
Maximum LED Drive Capability
Series
Parallel
Total
3
2
6
4
2
8
5
1
5
6
1
6
It is also important to note that for an input voltage supply which can be maintained at 5.0V or higher, the device
possesses even higher drive capabilities which can drive a higher number of parallel connected LEDs. The following table depicts the maximum LED numbers possible for this higher input voltage range.
1. Vin Variable Voltage of 5.0V~5.5V
2. LED forward voltage of 3.5V max.
Maximum LED Drive Capability
Rev 1.30
Series
Parallel
Total
3
3
9
4
2
8
5
1
5
6
1
6
10
November 9, 2010
HT7937
Package Information
6-pin SOT23-6 Outline Dimensions
D
C
L
H
E
q
e
A
A 2
b
Symbol
Dimensions in inch
Min.
Nom.
Max.
A
0.039
¾
0.051
A1
¾
¾
0.004
A2
0.028
¾
0.035
b
0.014
¾
0.020
C
0.004
¾
0.010
D
0.106
¾
0.122
E
0.055
¾
0.071
e
¾
0.075
¾
H
0.102
¾
0.118
L
0.015
¾
¾
q
0°
¾
9°
Symbol
Dimensions in mm
Min.
Nom.
Max.
1.00
¾
1.30
A1
¾
¾
0.10
A2
0.70
¾
0.90
A
Rev 1.30
A 1
b
0.35
¾
0.50
C
0.10
¾
0.25
D
2.70
¾
3.10
E
1.40
¾
1.80
e
¾
1.90
¾
H
2.60
¾
3.00
L
0.37
¾
¾
q
0°
¾
9°
11
November 9, 2010
HT7937
Product Tape and Reel Specifications
Reel Dimensions
D
T 2
A
C
B
T 1
SOT23-6
Symbol
Description
Dimensions in mm
A
Reel Outer Diameter
178.0±1.0
B
Reel Inner Diameter
62.0±1.0
C
Spindle Hole Diameter
13.0±0.2
D
Key Slit Width
T1
Space Between Flange
8.4
T2
Reel Thickness
11.4
Rev 1.30
2.50±0.25
12
+1.5/-0.0
+1.5/-0.0
November 9, 2010
HT7937
Carrier Tape Dimensions
P 0
D
P 1
t
E
F
W
B 0
C
D 1
P
K 0
A 0
R e e l H o le
IC
M a r k in g
SOT23-6
Symbol
Description
Dimensions in mm
W
Carrier Tape Width
8.0±0.3
P
Cavity Pitch
4.0±0.1
E
Perforation Position
1.75±0.1
F
Cavity to Perforation (Width Direction)
3.50±0.05
D
Perforation Diameter
1.5
+0.1/-0.0
D1
Cavity Hole Diameter
1.5
+0.1/-0.0
P0
Perforation Pitch
P1
Cavity to Perforation (Length Direction)
2.00±0.05
A0
Cavity Length
3.15±0.1
B0
Cavity Width
3.2±0.1
K0
Cavity Depth
1.4±0.1
t
Carrier Tape Thickness
C
Cover Tape Width
Rev 1.30
4.0±0.1
0.20±0.03
5.3±0.1
13
November 9, 2010
HT7937
Holtek Semiconductor Inc. (Headquarters)
No.3, Creation Rd. II, Science Park, Hsinchu, Taiwan
Tel: 886-3-563-1999
Fax: 886-3-563-1189
http://www.holtek.com.tw
Holtek Semiconductor Inc. (Taipei Sales Office)
4F-2, No. 3-2, YuanQu St., Nankang Software Park, Taipei 115, Taiwan
Tel: 886-2-2655-7070
Fax: 886-2-2655-7373
Fax: 886-2-2655-7383 (International sales hotline)
Holtek Semiconductor Inc. (Shenzhen Sales Office)
5F, Unit A, Productivity Building, No.5 Gaoxin M 2nd Road, Nanshan District, Shenzhen, China 518057
Tel: 86-755-8616-9908, 86-755-8616-9308
Fax: 86-755-8616-9722
Holtek Semiconductor (USA), Inc. (North America Sales Office)
46729 Fremont Blvd., Fremont, CA 94538
Tel: 1-510-252-9880
Fax: 1-510-252-9885
http://www.holtek.com
Copyright Ó 2010 by HOLTEK SEMICONDUCTOR INC.
The information appearing in this Data Sheet is believed to be accurate at the time of publication. However, Holtek assumes no responsibility arising from the use of the specifications described. The applications mentioned herein are used
solely for the purpose of illustration and Holtek makes no warranty or representation that such applications will be suitable
without further modification, nor recommends the use of its products for application that may present a risk to human life
due to malfunction or otherwise. Holtek¢s products are not authorized for use as critical components in life support devices
or systems. Holtek reserves the right to alter its products without prior notification. For the most up-to-date information,
please visit our web site at http://www.holtek.com.tw.
Rev 1.30
14
November 9, 2010