Rainbow Electronics MAX5095A User Manual
Page 14
MAX5094A/B/C/D/MAX5095A/B/C
To calculate the capacitance required, use the following
formula:
where:
I
G
= Q
G
f
SW
I
CC
is the MAX5094/MAX5095s’ maximum internal sup-
ply current after startup (see the Typical Operating
Characteristics to find the I
IN
at a given f
OSC
). Q
G
is the
total gate charge for the MOSFET, f
SW
is the converter
switching frequency, V
HYST
is the bootstrap UVLO hys-
teresis (0.8V), and t
SS
is the soft-start time, which is set
by external circuitry.
Size the resistor R
ST
according to the desired startup
time period, t
ST
, for the calculated C
ST
. Use the follow-
ing equations to calculate the average charging current
(I
CST
) and the startup resistor (R
ST
):
Where V
INMIN
is the minimum input supply voltage for
the application (36V for telecom), V
SUVR
is the bootstrap
UVLO wake-up level (8.4V), and I
START
is the V
IN
supply
current at startup (65µA, max). Choose a higher value for
R
ST
than the one calculated above if longer startup times
can be tolerated to minimize power loss in R
ST
.
The equation for C
ST
above gives a good approximation
of C
ST
, yet neglects the current through R
ST
. Fine tune
C
ST
using:
The above startup method is applicable to circuits where
the tertiary winding has the same phase as the output
windings. Thus, the voltage on the tertiary winding at any
given time is proportional to the output voltage and goes
through the same soft-start period as the output voltage.
The minimum discharge time of C
ST
from 8.4V to 7.6V
must be greater than the soft-start time (t
SS
).
Undervoltage Lockout (UVLO)
The minimum turn-on supply voltage for the
MAX5094/MAX5095 is 8.4V. Once V
CC
reaches 8.4V,
the reference powers up. There is 0.8V of hysteresis
from the minimum turn-on voltage to the UVLO thresh-
old. Once V
CC
reaches 8.4V, the MAX5094/MAX5095
operates with V
CC
down to 7.6V. Once V
CC
goes below
7.6V the device is in UVLO. When in UVLO, the quies-
cent supply current into V
CC
falls back to 32µA (typ),
and OUT and REF are pulled low.
MOSFET Driver
OUT drives an external n-channel MOSFET and swings
from GND to V
CC
. Ensure that V
CC
remains below the
absolute maximum V
GS
rating of the external MOSFET.
OUT is a push-pull output with the on-resistance of the
PMOS typically 3.5
Ω and the on-resistance of the NMOS
typically 4.5
Ω. The driver can source 2A typically and
sink 1A typically. This allows for the MAX5094/MAX5095
to quickly turn on and off high gate-charge MOSFETs.
Bypass V
CC
with one or more 0.1µF ceramic capacitors
to GND, placed close to the MAX5094/MAX5095. The
average current sourced to drive the external MOSFET
depends on the total gate charge (Q
G
) and operating
frequency of the converter. The power dissipation in the
MAX5094/MAX5095 is a function of the average output-
drive current (I
DRIVE
). Use the following equation to cal-
culate the power dissipation in the device due to I
DRIVE
:
I
DRIVE
= Q
G
x f
SW
PD = (I
DRIVE
+ I
CC
) x V
CC
where, I
CC
is the operating supply current. See the
Typical Operating Characteristics for the operating
supply current at a given frequency.
Error Amplifier (MAX5094)
The MAX5094 includes an internal error amplifier. The
inverting input is at FB and the noninverting input is inter-
nally connected to a 2.5V reference. The internal error
amplifier is useful for nonisolated converter design (see
Figure 6) and isolated design with primary-side regulation
through a bias winding (see Figure 5). In the case of a
nonisolated power supply, the output voltage is:
where, R1 and R2 are from Figure 6.
V
R
R
V
OUT
= +
×
1
1
2
2 5
.
C
I
I
V
V
R
V
t
ST
CC
G
INMIN
ST
HYST
SS
=
+ −
−
8
(
)
R
V
V
I
I
ST
INMIN
SUVR
CST
START
≅
−
+
2
I
V
C
t
CST
SUVR
ST
ST
=
×
C
I
I
t
V
ST
CC
G
SS
HYST
=
+
[
]
( )
High-Performance, Single-Ended, Current-Mode
PWM Controllers
14
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