Rainbow Electronics MAX1567 User Manual
Page 30
MAX1566/MAX1567
Six-Channel, High-Efficiency, Digital
Camera Power Supplies
30
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If the output filter capacitor has significant ESR, a zero
occurs at the following:
Z
ESR
= 1 / (2
π x C
OUT
x R
ESR
)
If Z
ESR
> f
C
, it can be ignored, as is typically the case
with ceramic output capacitors. If Z
ESR
< f
C
, it should
be cancelled with a pole set by capacitor C
P
connect-
ed from C
C
to GND:
C
P
= C
OUT
x R
ESR
/ R
C
If C
P
is calculated to be <10pF, it can be omitted.
AUX Controller Component Selection
External MOSFET
All MAX1566/MAX1567 AUX controllers drive external
logic-level MOSFETs. Significant MOSFET selection
parameters are as follows:
• On-resistance (R
DS(ON)
)
• Maximum drain-to-source voltage (V
DS(MAX)
)
• Total gate charge (Q
G
)
• Reverse transfer capacitance (C
RSS
)
On the MAX1566, all AUX drivers are designed for N-
channel MOSFETs. On the MAX1567, AUX2 is a DC-to-
DC inverter, so DL2 is designed to drive a P-channel
MOSFET. In both devices, the driver outputs DL1 and
DL3 swing between PVSU and GND. MOSFET driver
DL2 swings between INDL2 and GND.
Use a MOSFET with on-resistance specified with gate
drive at or below the main output voltage. The gate
charge, Q
G
, includes all capacitance associated with
charging the gate and helps to predict MOSFET transi-
tion time between on and off states. MOSFET power
dissipation is a combination of on-resistance and tran-
sition losses. The on-resistance loss is as follows:
P
RDSON
= D x I
L
2
x R
DS(ON)
where D is the duty cycle, I
L
is the average inductor
current, and R
DS(ON)
is MOSFET on-resistance. The
transition loss is approximately as follows:
P
TRANS
= (V
OUT
x I
L
x f
OSC
x t
T
) / 3
where V
OUT
is the output voltage, I
L
is the average
inductor current, f
OSC
is the switching frequency, and
t
T
is the transition time. The transition time is approxi-
mately Q
G
/ I
G
, where Q
G
is the total gate charge, and
I
G
is the gate-drive current (0.5A typ). The total power
dissipation in the MOSFET is as follows:
P
MOSFET
= P
RDSON
+ P
TRANS
Diode
For most AUX applications, a Schottky diode rectifies
the output voltage. Schottky low forward voltage and
fast recovery time provide the best performance in
most applications. Silicon signal diodes (such as
1N4148) are sometimes adequate in low-current
(<10mA), high-voltage (>10V) output circuits where the
output voltage is large compared to the diode forward
voltage.
AUX Compensation
The auxiliary controllers employ voltage-mode control
to regulate their output voltage. Optimum compensa-
tion depends on whether the design uses continuous or
discontinuous inductor current.
AUX Step-Up, Discontinuous Inductor Current
When the inductor current falls to zero on each switch-
ing cycle, it is described as discontinuous. The inductor
is not utilized as efficiently as with continuous current,
but in light-load applications this often has little negative
impact since the coil losses may already be low com-
pared to other losses. A benefit of discontinuous induc-
tor current is more flexible loop compensation, and no
maximum duty-cycle restriction on boost ratio.
To ensure discontinuous operation, the inductor must
have a sufficiently low inductance to fully discharge on
each cycle. This occurs when:
L < [V
IN
2
(V
OUT
- V
IN
) / V
OUT
3
] [R
LOAD
/ (2f
OSC
)]
A discontinuous current boost has a single pole at the
following:
f
P
= (2V
OUT
- V
IN
) / (2
π x R
LOAD
x C
OUT
x V
OUT
)
Choose the integrator cap so the unity-gain crossover,
f
C
, occurs at f
OSC
/ 10 or lower. Note that for many AUX
circuits, such as those powering motors, LEDs, or other
loads that do not require fast transient response, it is
often acceptable to overcompensate by setting f
C
at
f
OSC
/ 20 or lower.
C
C
is then determined by the following:
C
C
= [2V
OUT
x V
IN
/ ((2V
OUT
- V
IN
) x V
RAMP
)] [V
OUT
/
(K(V
OUT
- V
IN
))]
1/2
[(V
FB
/ V
OUT
)(g
M
/ (2
π x f
C
))]
where:
K = 2L x f
OSC
/ R
LOAD
and V
RAMP
is the internal slope-compensation voltage
ramp of 1.25V.