DN54

Design Note 54
Issue 1 December 2000
High Efficiency DC/DC Converter, using High
Density SOT23-6 MOSFETs.
Introduction.
Circuit Operation.
Portable applications such as laptop
computers, mobile phones, digital
cameras, video cameras, etc. require
high efficiency DC/DC converters to
extend battery life and give the
manuf actur ers an edge in a very
competitive marketplace. This design
note shows a typical power converter
application with improved overall
circuit performance using a device from
t h e Z e t e x r a n g e o f H i g h D e n si t y
MOSFETs, the ZXM62N02E6.
The circuit in Figure 1. employs a boost
topology combined with a Maxim
controller IC. The energy stored in L1
during Q1 on time is delivered to the
output via D1 when Q1 turns off and the
polarity across L1 reverses. During Q1
o n t i me t he ou t pu t ca pa ci t or ,C 4,
supplies the output current and when
Q1 is turned off the stored energy in C4
is recharged. This energy, together with
the input current, supplies the load
current.
Features.
• High efficiency >85%.
• Up to 5W output power.
• HDMOS Technology - loss reduction.
• SOT23-6 packaging, reducing
physical size.
The MAX608 IC is used to control the
switching of the transistor to maintain
the output voltage regulation over a
wide range of load currents. The
MAX608 employs a current-limited
Pulse Frequency Modulation, PFM,
scheme that combines the low
quiescent current of PFM with the load
driving capability of PWM. The current
sense resistor, R1, monitors the output.
INPUT 2V TO 5.5V
OUTPUT 5V upto 1A
OFF
EXT
CS
V+
1
8
2
3 FB
7 GND
6 AGND
4
SHDN
ON 5 REF
Figure 1.
H igh
Eff iciency
DC/DC Controller.
DN54-1
Design Note 54
Issue 1 December 2000
Circuit Performance.
Input voltages can range from 2V to 5.5V
for a 5V output. Up to 1A output current
can be achieved, maintaining a line
regulation of +/-2% with a good EMC
signature (see Figure 2). By utilising a
Zetex
HDMOS
MOSFET,
the
ZXM62N02E6, as the main power switch
in the converter, the major losses in the
circuit can be significantly reduced (see
Figure 3).
Figure 3.
’On-state’ losses in Q1.
100
ZXM62N02E6 - Vin= 3.3V
Efficiency (%)
95
Figure 2.
Output Voltage Ripple.
90
85
80
75
The fast switching characteristics of the
ZXM62N02E6 lower switching losses
which, coupled with a low RDS(on) value,
give a reduction in ’on-state’ losses.
This, combined with the Zetex
ZHCS1000 1A DC rated Schottky diode
f u r t he r r educe circuit losses and
therefore increase efficiency to >85%
(see Figure 4).
0
0.5
Iout (A)
Figure 4.
Efficiency v Output Current.
DN 54- 2
1
Design Note 54
Issue 1 December 2000
S urf ace mount packaging for the
ZXM62N02 includes SOT23-6 and SO8.
These packages ensure a high power
output to small physical size ratio, up to
5 watts, which is critical in portable
applications.
For lower output currents (up to 0.5A)
the ZXM61N02F, available in SOT23, is
an option. A performance comparison
bet w een the S OT23 and SOT23-6
package options is shown below.
Part Number
Package
ID (max)
RDS(on)@
VGS=4.5V
Efficiency
ZXM62N02E6
SOT23-6
3.2A
[email protected]
[email protected]
ZXM61N02F
SOT23
1.5A
[email protected]
[email protected]
DN54-3
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