WEITRON
2A Step-down Voltage Regulator
P b
Lead(Pb)-Free
Power Converter 150kHz
General Description:
WT2595M
The WT2595M is a monolithic integrated circuit that provide all the active functions for a step-down
switching regulator, capable of driving a 2A load without additional transistor component. Requiring
a minimum number of external component, the board space can be saved easily. The external
shutdown function can be controlled by TTL logic level and then come into standby mode. The
internal compensation makes feedback control have good line and load regulation without external
design. Regarding protected function, thermal shutdown is to prevent over temperature operating
from damage, and current limit is against over current operating of the output switch. The WT2595M
operates at a switching frequency of 150Khz thus allowing smaller sized filter components than what
would be needed with lower frequency switching regulators. Other features include a guaranteed
+4% tolerance on output voltage under specified input voltage and output load conditions, and +15%
on the oscillator frequency. The output version included fixed 3.3V, 5V, 12V, and an adjustable
type. The packages are available in a standard 8-lead SOP8.
Features:
* 3.3V, 5V, 12V and adjustable output versions
* Adjustable version output voltage range, 1.23V to 37V +4% max over line and load condiction
* SOP-8 packages
* Voltage mode non-synchronous PWM control
* Thermal-shutdown and current-limit protection
* ON/OFF shutdown control input
* Input voltage range up to 40V
* Output load current: 2A
* 150 kHz fixed frequency internal oscillator
* Low power standby mode
* Built-in switching transistor on chip
Applications:
* Simple High-efficiency step-down(buck) regulator
* Efficient preregulator for linear regulators
* On-card switching regulators
* Positive to negative converter
* Battery Charger
WEITRON
hpp://www.weitron.com.tw
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28-Sep-06
WT2595M
Typical Application Circuit
FB
DC Input
Vin
1
12V
WT2595M
-33
4
8
SD
7 6 5
3
2
Out
L1
33uH
3.3V/2A
Output Load
Inductor
Capacitor
C
in
Gnd
Schottky Diode
D1
Capacitor
Co
Function Description
Pin Functions
+V
IN
This is the positive input supply for the IC switching regulator. A suitable input bypass capacitor must be
present at this pin to minimize voltage transients and to supply the switching currents needed by the regulator.
Out
Internal switch and power output. The voltage at this pin switches between (+V
IN
– V
SAT
) and approximately –
0.5V, with a duty cycle of approximately V
OUT
/ V
IN
. The PC board copper area connected to this pin should be
kept a minimum in order to reduce the coupling sensitivity to the circuitry
Ground
Circuit ground.
Feedback
Complete the feedback loop by sensing the regulated output voltage
ON
/OFF
Allows the switching regulator circuit to be shutdown using logic level signals thus dropping the total input
supply current to approximately 100uA. Pulling this pin below a threshold voltage of approximately 1.3V
turns the regulator on, and pulling this pin above 1.3V (up to a maximum of 25V) shuts the regulator down.
If this shutdown feature is not needed, the
ON /OFF pin must be wired to the ground pin, in either case the
regulator will be in the ON condition.
Thermal Considerations
The SOP-8 package needs a heat sink under most condictions . The size of the heatsink depends on
the input voltage, the output voltage, the load current and the ambient temperature. The WT2595M
junction temperature rises above ambient temperature for a 2A load and different input and output voltages.
The data for these curves was taken with the WT2595M operating as a buck switching regulator in an ambient
temperature of 25
o
C (still air). These temperature increments are all approximate and are affected by many
factors. Some of these factors include board size, shape ,thickness ,position ,location, and even board
temperature. Other factors are trace width, total printed circuit copper area, copper thickness , single or
double-sided, multi-layer board and amount of solder on the board. Higher ambient temperatures require
more heat sinking.
For the best thermal performance ,wide copper traces and generous amounts of printed circuit board copper
should be used in the board layout. (One exception is the out(switch) pin, which should not have large areas
of copper.) Large areas of copper provide the best transfer of heat(lower thermal resistance) to the
surrounding air, and moving air lowers the thermal resistance even further.
The effectiveness of the PC board to dissipate heat also depends on the size, quantity and spacing of other
components on the board , as well as whether the surrounding air is still or moving. Furthermore, some of
these components such as the catch diode will add heat to the PC board and heat can vary as the input
voltage changes. For the inductor, depending on the phical size, type of core material and the DC resistance,
it could either act as a heat sink taking heat away from the board, or it could add heat to the board.
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WEITRON
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WT2595M
Electrical Characteristics
Unless otherwise specified, V
IN
=12V for 3.3V, 5V, adjustable version and V
IN
=24V for the 12V version. I
LOAD
= 0.2A
Symbol
I
B
F
OSC
Fscp
V
SAT
DC
I
CL
I
L
I
Q
I
STBY
V
IL
V
IH
I
H
I
L
Ts
Parameter
Feedback bias current
oscillator frequency
Oscillator frequency
short circuit protect
saturation voltage
Max. Duty Cycle(ON)
Min. Duty cycle(OFF)
current limit
Output = 0
Output = 1
Output
leakage
current
Conditions
V
FB
=1.3V
(Adjustable version only)
Min.
Typ.
-10
Max.
-50
-100
173
173
70
1.4
1.5
Unit
nA
Khz
Khz
V
%
127
of When current limit occurred
and V
FB
<0.55V
I
OUT
=1.5A
no outside circuit
V
FB
=0V force driver on
V
FB
=0V force driver on
V
FB
=12V force driver off
peak current
no outside circuit
V
FB
=0 force driver on
no outside circuit
V
FB
=12 force driver off
V
IN
=24V
V
FB
=12 force driver off
ON/OFF pin=5V
V
IN
=24V
Low (regulator ON)
High (regulator OFF)
-
2.0
110
150
30
1.25
100
0
2.4
2.8
3.3
3.6
-200
A
uA
mA
mA
uA
-5
5
70
10
150
200
0.6
1.3
-
-0.01
-0.1
175
150
-1
Quiescent Current
Standby Quiescent
Current
ON/OFF pin logic input
threshold voltage
V
ON/OFF pin logic
V
LOGIC
=2.5V (OFF)
input current
ON/OFF pin input current V
LOGIC
=0.5V (ON)
Over temperature
shutdown threshold
Tj increasing
Tj decreasing
uA
°C
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WEITRON
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