SAMHOP Microelectronics Corp.
1.2MHz STEP-UP CONVERTER
OPERATION
The SM8237 uses a constant frequency, current mode control scheme to provide excellent line and load
regulation. Operation can be best understood by referring to the block diagram in figure 1. At the start of each
oscillator cycle, the SR latch is set, which turns on the power switch Q1. A voltage proportional to the switch
current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal pf the PWM
comparator A2. When this voltage exceeds the level at the negative input of A2, the SR latch is reset turning off
the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an
amplified version of the difference between the feedback voltage and the reference voltage of 95mV. In this
manner, the error amplifier sets the correct peak current level to keep the output in regulation. If the error
amplifier's output increases, more current is delivered to the output; if it decreases, less current is delivered.
Minimum output current
The SM8237 can regulate three series LEDs connected at low output currents, down to approximately 4mA
from a 4.2V supply, without pulse skipping, using the same external components as specified for 15mA
operation. As current is further reduced, the device will begin skipping pulses. This will result in some low
frequency ripple, although the LED current remains regulated on an average basis down to zero.
APPLICATIONS INFORMATION
Inductor selection
A 22u H inductor is recommended for most SM8237 applications. Although small size and high efficiency are
major concerns, the inductor should have low core losses at 1.2MHz and low DCR (copper wire resistance).
Some inductors in this category with small size are listed in Table 1.
Capacitor selection
The small size of ceramic capacitors makes them ideal for SM8237 applications. X5R and X7R types are
recommended because they retain their capacitance over wider voltage and temperature ranges than other
types such as Y5V or Z5U. A 1μF input capacitor and a 0.22μF output capacitor are sufficient for most of
SM8237 applicaiotns.
Diode selection
Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for
SM8237 applications. The forward voltage drop of a Schottky diode represents the conduction losses in the
diode, while the diode capacitance (C
T
or C
D
) represents the switching losses. For diode selection, both
forward voltage drop and diode capacitance need to be considered. Schottky diodes with higher current ratings
usually have lower forward voltage drop and larger diode capacitance, which can cause significant switching
losses at the 1.2MHz switching frequency of the SM8237. A Schottky diode rated at 100mA to 200mA is
sufficient for most SM8237 applications.
Part number
LQH3C220
ELJPC220KF
LB2012B220M
LEM2520-220
DCR
(Ω)
0.71
4.0
1.7
5.5
Current rating
(mA)
250
160
350
75
125
Manufacturer
Murata
Panasonic
Sumida
Taiyo Yuden
Taiyo Yuden
BAT54
200
Part
number
CMDSH-3
CMDSH2-3
Forward
current (mA)
100
100
Voltage drop Diode
Manufacturer
capacitance (pF)
(V)
0.58 at
100mA
0.49 at
200mA
0.53 at
100mA
7.0 at 10V
15 at 10V
10 at 25V
Central
Central
Zetex
CDRH3D16-220 0.53
Table 1. Recommended inductors
Table 2. Recommended Schottky diodes
Page 8
V.1.3 Aug 28,2007
SAMHOP [ SAMHOP MICROELECTRONICS CORP. ]
