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HT7938 High Current and Performance White LED Driver
Features
* Efficiency up to 85% at VIN=3.6V, 9LEDs, * Tiny inductor and capacitors * Up to 10 strings White-LED (LED VF(Max.)=3.5V) * Tiny 6-lead SOT23-6 package * Built in OVP, OCP, OTP, UVLO protection
ILED=20mA
* 1.2MHz fixed switching frequency * Low Standby Current: 0.1mA, VEN=0V (typ.) * Matches LED current
Applications
* Cellular phones * PDAs * DSCs * GPS * Handheld devices * White LED display backlighting
General Description
HT7938 is high efficiency boost converter with constant current output to provide backlight in handheld devices. Series connection of LEDs provides constant identical LED currents resulting in uniform brightness. The continuous LED current is set with FB pin regulated voltage across an external sense resistor (RFB) connected from that pin to ground. The built-in open load protection prevents the damage resulting from an open circuit condition. Low 200mV feedback voltage minimizes power loss in the current setting resistor for better efficiency. HT7938 switches at rates up to 1.2MHz to allow the use of extremely small inductor and filter capacitor.
Block Diagram
Rev 1.10
1
March 19, 2010
HT7938
Pin Assignment
V IN 6 OVP 5 EN 4 T o p V ie w
H T7938 S O T 2 3 -6 -A 1 SW 2 GND 3 FB
Pin Description
Pin Name EN GND I/O I 3/4 Description Shutdown & Dimming control input. Dont allow this pin to float. Signal Ground. Feedback pin. Reference voltage. The HT7938 feedback voltage is 200mV. Connect the sense resistor from FB to GND to set the LED current. Calculate resistor value according to 200mV . RFB = ILED Switching pin. Internal power MOSFET drain. Connected to inductor and diode. Input supply pin. The input supply pin for the IC. Connect VIN to a supply voltage between 2.6V~5.5V. Over voltage protection pin which is connected to the output.
FB
I
SW VIN OVP
I I O
Absolute Maximum Ratings
Input Voltage...........................................................6.0V FB Voltage ..............................................................6.0V OVP Voltage ............................................................46V Storage Temperature Range ..............-55C to +150C SW Voltage..............................................................46V EN ..........................................................................6.0V Operating Temperature Range .............-40C to +85C Maximum Junction Temperature........................+150C
Note: These are stress ratings only. Stresses exceeding the range specified under Absolute Maximum Ratings 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.
Rev 1.10
2
March 19, 2010
HT7938
Electrical Characteristics
Symbol VIN ULVO Parameter Input Voltage Under Voltage Lockout 3/4 3/4 Switching, VFB=GND IIN Supply Current VEN= 0V Not switching, VFB=0.25V Error Amplifier VFB Feedback Voltage 3/4 190 200 210 mV VIN=3.6V, L=10mH, CIN=1mF, CO=1mF, ILED=20mA, Ta=25C Test Conditions Min. 2.6 3/4 3/4 3/4 3/4 Typ. 3/4 2.4 1.0 0.1 200 Max. 5.5 2.5 2.5 1.0 300 Unit V V mA mA mA
Power Switch fOSC DC RDS(ON) ISW(OFF) EN Pin VIH VIL EN Voltage High EN Voltage Low VIN=2.6V~5.5V VIN=2.6V~5.5V 2.0 3/4 3/4 3/4 3/4 0.8 V V Switching Frequency Measurement at SW pin Maximum Duty Cycle SW On Resistance Switch Leakage Current 3/4 3/4 92 3/4 3/4 95 0.7 0.1 0.8 1.2 1.6 3/4 3/4 1.0 MHz % W mA
OVP and OCP VOVP IOCP OVP Threshold N-channel MOSFET Current Limit No load 3/4 36 3/4 39 650 42 3/4 V mA
Thermal Shutdown TSHUT Thermal Shutdown Threshold Thermal Shutdown Hysteresis 3/4 3/4 3/4 3/4 150 15 3/4 3/4 C C
Rev 1.10
3
March 19, 2010
HT7938
Function Description
VIN Under-Voltage Lockout - UVLO The device contains an Input Under Voltage Lockout (UVLO) circuit. The purpose of the UVLO circuit is to ensure that the input voltage is high enough for reliable operation. When the input voltage falls below the under voltage threshold, the internal FET switch is turned off. If the input voltage rises by the under voltage lockout hysteresis, the device will restart. The UVLO threshold is set below the minimum input voltage of 2.6V to avoid any transient VIN drops under the UVLO threshold and causing the converter to turn off. Current Limit Protection The device has a cycle-by-cycle current limit to protect the internal power MOSFET. If the inductor current reaches the current limit threshold, the MOSFET will be turned off. It is import to note that this current limit will not protect the output from excessive current during an output short circuit. If an output short circuit has occurred, excessive current can damage both the inductor and diode. Over-Voltage Protection - OVP The device provides an over-voltage protection function. If the FB pin is shorted to ground or an LED is disconnected from the circuit, the FB pin voltage will fall to zero and the internal power MOSFET will switch with its full duty cycle. This may cause the output voltage to exceed its maximum voltage rating, possibly damaging the IC and external components. Internal over-voltage protection circuitry turns off the power MOSFET and shuts down the IC as soon as the output voltage exceeds the VOVP threshold. As a result, the output voltage falls to the level of the input supply voltage. The device remains in shutdown mode until the power is recycled. Over-Temperature protection - OTP A thermal shutdown is implemented to prevent damages due to excessive heat and power dissipation. Typically the thermal shutdown threshold is 150C. When the thermal shutdown is triggered the device stops switching until the temperature falls below typically 135C. Then the device starts switching again. Application Information
* Inductor Selection
I
IN (M A X )
Choose an inductor that can handle the necessary peak current without saturating, and ensure that the inductor has a low DCR to minimise power losses. A 10mH~22mH inductor should be a good choice for most HT7938 applications. However, a more exact inductance value can be calculated. A good rule for choosing an inductor value is to allow the peak-to-peak ripple current to be approximately 30~50% of the maximum input current. Calculate the required inductance value using the following equation:
L= V = V
IN
(V F
OUT
OUT SW
-V
IN
)
L
OUT
DI
V
I V
IN
O U T (M A X )
h
IN (M A X )
DI
L
= (3 0 % =I
~ 50% )I
IN (M A X )
I
L (P E A K )
+
1 2
D IL
In the equation above, IOUT(MAX) is the maximum load current, DIL is the peak-to-peak inductor ripple current, h is the converter efficiency, FSW is the switching frequency and IL(PEAK) is the peak inductor current.
* Output Capacitor Selection
The output capacitor determines the steady state output voltage ripple. The voltage ripple is related to the capacitors capacitance and its ESR (Equivalent Series Resistance). A ceramic capacitor with a low ESR value will provide the lowest voltage ripple and are therefore recommended. Due to its low ESR, the capacitance value can be calculated by the equation:
C
out
= V
(V
O
-V F
IN SW
) IO
UT r ip p le
OUT
V
In the equation above, Vripple =peak to peak output ripple, FSW is the switching frequency. A 1mF~10mF ceramic capacitor is suitable for most application.
* Input Capacitor Selection
The selection of the inductor affects steady state operation as well as transient behavior and loop stability. There are three important electrical parameters which need to be considered when choosing an inductor: the value of inductor, DCR (copper wire resistance) and the saturation current.
An input capacitor is required to supply the ripple current to the inductor, while limiting noise at the input source. A low ESR ceramic capacitors is required to keep the noise at the IC to a minimum. A 4.7mF~10mF ceramic capacitor is suitable for most application. This capacitor must be connected very close to the VIN pin and inductor, with short traces for good noise performance.
Rev 1.10
4
March 19, 2010
HT7938
* Schottky Diode Selection
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 minimize EMI and switching noise, please follow the guidelines below:
* All tracks should be as wide as possible. * The input and output capacitors, C1 and C2, should
The output rectifier diode conducts during the internal MOSFET is turn off. The average and peak current rating must be greater than the maximum output current and peak inductor current. The reverse breakdown voltage must be greater than the maximum output voltage. It is recommended to use a schottky diode with low forward voltage to minimize the power dissipation and therefore to maximize the efficiency of the converter. A 1N5819 type diode is recommended for HT7938 applications.
* LED Current Selection
be placed close to the VIN, VO and GND pins.
* The Schottky diode, D1, and inductor, L, must be
placed close to the SW pin.
* Feedback resistor, Rfb, must be placed close to the
The LED current is controlled by the current sense feedback resistor Rfb, The current sense feedback reference voltage is 200mV. In order to have accurate LED currents, precision resistors are the preferred type with a 1% tolerance. The LED current can be calculated from the following formula.
I
LED
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.
=
V R
FB fb
=
200m V R fb
Where ILED is the total output LED current, VFB=feedback voltage, Rfb=current sense resistor.
* Digital and Analog Dimming Control
The LED illumination level can be controlled using both digital and analog methods. The digital method uses a PWM signal applied to the EN pin. This is shown in figure 8. The average LED current increases proportionally with the PWM signal duty cycle. A 0% duty cycle corresponds to zero LED current. A 100% duty cycle corresponds to full LED current. The PWM signal frequency should be set below 1kHz due to the delay time of device startup. There are two methods of analog LED brightness control. The first method uses a DC voltage to control the feedback voltage. If the DC voltage range is from 0V to 3.3V, the selection of resistors control the LED current from 20mA to 0mA. The other way is to use a filtered PWM signal, as shown in figure 10. The filtered PWM signal application acts in the same way as the DC voltage dimming control.
Top Layer
Bottom Layer Rev 1.10 5 March 19, 2010
HT7938
Typical Performance Characteristics
Figure 1. Efficiency vs Input Voltage
Figure 5. LED Current vs PWM Dimming (10S1P LEDs)
Figure 2. Enable Voltage vs. Input Voltage
Figure 6. LED Current vs PWM Dimming (9S1P LEDs)
Figure 3. Operating current vs. Input Voltage Figure 7. Switching Frequency (MHz) vs. Input Voltage
Figure 4. Reference Voltage vs. Input Voltage
Rev 1.10
6
March 19, 2010
HT7938
Application Circuits
V IN 2 .6 V ~ 5 .5 V C IN 1mF V IN EN
PW M S ig n a l
10mH
1N5819 CO 1mF 10 LED s OVP FB R
FB
SW
GND H T7938
10W
Figure 8. Application Circuits for Driving 10S1P WLEDs
V IN 2 .6 V ~ 5 .5 V C IN 1mF V IN EN
10mH
1N5819 CO 1mF 10 LED s OVP FB 10kW R 150kW
FB
SW
GND H T7938
10W
VDC 0 V ~ 3 .3 V
Figure 9. Application Circuit for Dimming Control Using a DC Voltage
V IN 2 .6 V ~ 5 .5 V C IN 1mF V IN EN
10mH
1N5819 CO 1mF 10 LED s OVP FB 10kW R 150kW 10kW 0 .1 m F
FB
SW
GND H T7938
3 .3 V 0V PW M
10W
S ig n a l
Figure 10. Application Circuit for Dimming Control Using a Filtered PWM Signal
Rev 1.10
7
March 19, 2010
HT7938
Package Information
6-pin SOT23-6 Outline Dimensions
D
C
L
E
H
e
A2 b A1
A
Symbol A A1 A2 b C D E e H L q Symbol A A1 A2 b C D E e H L q
Dimensions in inch Min. 0.039 3/4 0.028 0.014 0.004 0.106 0.055 3/4 0.102 0.015 0 Nom. 3/4 3/4 3/4 3/4 3/4 3/4 3/4 0.075 3/4 3/4 3/4 Dimensions in mm Min. 1.00 3/4 0.70 0.35 0.10 2.70 1.40 3/4 2.60 0.37 0 Nom. 3/4 3/4 3/4 3/4 3/4 3/4 3/4 1.90 3/4 3/4 3/4 Max. 1.30 0.10 0.90 0.50 0.25 3.10 1.80 3/4 3.00 3/4 9 Max. 0.051 0.004 0.035 0.020 0.010 0.122 0.071 3/4 0.118 3/4 9
Rev 1.10
8
q
March 19, 2010
HT7938
Product Tape and Reel Specifications
Reel Dimensions
T2 D
A
B
C
T1
SOT23-6 Symbol A B C D T1 T2 Description Reel Outer Diameter Reel Inner Diameter Spindle Hole Diameter Key Slit Width Space Between Flange Reel Thickness Dimensions in mm 178.01.0 62.01.0 13.00.2 2.500.25 8.4 11.4
+1.5/-0.0 +1.5/-0.0
Rev 1.10
9
March 19, 2010
HT7938
Carrier Tape Dimensions
D
E F
P0
P1
t
W C
B0
D1
P A0
K0
R e e l H o le IC p a c k a g e p in 1 a n d th e r e e l h o le s a r e lo c a te d o n th e s a m e s id e .
SOT23-6 Symbol W P E F D D1 P0 P1 A0 B0 K0 t C Description Carrier Tape Width Cavity Pitch Perforation Position Cavity to Perforation (Width Direction) Perforation Diameter Cavity Hole Diameter Perforation Pitch Cavity to Perforation (Length Direction) Cavity Length Cavity Width Cavity Depth Carrier Tape Thickness Cover Tape Width Dimensions in mm 8.00.3 4.00.1 1.750.1 3.500.05 1.5 1.5
+0.1/-0.0 +0.1/-0.0
4.00.1 2.000.05 3.150.10 3.20.1 1.40.1 0.200.03 5.30.1
Rev 1.10
10
March 19, 2010
HT7938
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, USA Tel: 1-510-252-9880 Fax: 1-510-252-9885 http://www.holtek.com
Copyright O 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. Holteks 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 1.10
11
March 19, 2010

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