Rainbow Electronics MAX742 - Manual (Page 11)

MAX742
Switch-Mode Regulator with
+5V to ±12V or ±15V Dual Output
______________________________________________________________________________________ 11
where:
V SW is the voltage drop across the the switch transistor
and current-sense resistor in the on state (0.3V typ).
V D is the rectifier forward voltage drop (0.4V typ).
LIR is the ratio of peak-to-peak ripple current to DC
offset current in the inductor (0.5 typ).
Current-Sense Resistor Value
The current-sense resistor values are calculated accord-
ing to the worst-case-low current-limit threshold voltage
from the
Electrical Characteristics table and the peak
inductor current. The peak inductor current calculations
that follow are also useful for sizing the switches and
specifying the inductor current saturation ratings.
150mV
R SENSE = ————
I PEAK
I LOAD (V OUT + V D )
+I PEAK (boost) = ————————— +
V
IN
- V
SW
(V IN - V SW ) (V OUT + V D - V IN )
—————————————
(2)(F)(L)(V
OUT
+ V
D
)
I
LOAD
(V
OUT
+ V
D
+ V
IN
)
+I PEAK (inverting) = ———————————— +
V IN - V SW
(V
IN
- V
SW
) (V
OUT
+ V
D
+ V
IN
)
—————————————
(2)(F)(L) (V OUT + V D )
Filter Capacitor Value
The output filter capacitor values are generally deter-
mined by the effective series resistance (ESR) and volt-
age rating requirements rather than actual capacitance
requirements for loop stability. In other words, the
capacitor that meets the ESR requirement for noise pur-
poses nearly always has much more output capaci-
tance than is required for AC stability. Output voltage
noise is dominated by ESR and can be roughly calcu-
lated by an Ohm’s Law equation:
V NOISE (peak-to-peak) = I PEAK x R ESR
where V NOISE is typically 0.15V.
Ensure the output capacitors selected meet the follow-
ing minimum capacitance requirements:
Minimum CF = 60µF per output or the following, whichev-
er is greater:
CF = 0.015/R LOAD (in Farads, ±15V mode)
CF = 0.01/R LOAD (in Farads, ±12V mode)
Compensation Capacitor (CC) Value
The compensation capacitors (CC+ and CC-) cancel
the zero introduced by the output filter capacitors’ ESR,
improving phase margin, and AC stability. The com-
pensation poles set by CC+ and CC- should be set to
match the ESR zero frequencies of the output filter
capacitors according to the following:
R ESR x CF
CC (in Farads) = —————— (use 1000pF minimum)
10k
Ω
Standard 6W Application
The 6W supply (Figure 2) generates ±200mA at ±15V,
or ±250mA at ±12V. Output capability is increased to
10W or more by heatsinking the power FETs, using
cores with higher current capability (such as Gowanda
#050AT1003), and using higher filter capacitance.
Ferrite and MPP inductor cores optimize efficiency and
size. Iron-power toroids designed for high frequencies
are economical, but larger.
Ripple is directly proportional to filter capacitor equiva-
lent series resistance (ESR). In addition, about 250mV
transient noise occurs at the LX switch transitions. A
very short scope probe ground lead or a shielded
enclosure is need for making accurate measurements
of transient noise. Extra filtering, as shown in Figure 2,
reduces both noise components.
High-Power 22W Application
The 22W application circuit (Figure 3) generates ±15V
at ±750mA or ±12V at ±950mA. Noninductive wire-
wound resistors with Kelvin current-sensing connec-
tions replace the metal-film resistors of the previous
(6W) circuit. Gate drive for the P-channel FET is boot-
strapped from the negative supply via diode D6. The
2.7V zener (D5) is required in 15V mode to prevent
overvoltage. The charge pump (D3, D4, and C6) may
not be necessary if the circuit is lightly loaded
(<100mA) on start-up. AIE part #415-0963 is a ferrite
pot-core inductor that can be used in place of a small-
er, more expensive moly-permalloy toroid inductor (L1,
L2). Higher efficiencies can be achieved by adding
extra MOSFETs in parallel. Load levels above 10W
make it necessary to add heatsinks, especially to the P-
channel FET.