MAX1565ETJ-T Maxim Integrated Products, MAX1565ETJ-T Datasheet - Page 20

• Manufacturer Part Number: MAX1565ETJ-T
• Description: DC/DC Switching Converters 5Ch Digital Camera Power Supply
• Manufacturer: Maxim Integrated Products

If we select L = 4.7µH and f
V
Choose 3.3nF. Now select R
requirements are met. For example, if 4% transient
droop is allowed, the input to the error amplifier moves
0.04 x 1.25V, or 50mV. The error amp output drives
50mV x 135µS, or 6.75µA across R
sient gain. Since the current-sense transresistance is
0.6V/A, the value of R
step swing:
In a step-down DC-to-DC converter, if L
output current relates to inductor current by:
Thus, for a 250mA output load step with V
and V
Choose 27kΩ. Note that the inductor does not limit the
response in this case since it can ramp at (V
V
The output filter capacitor is then chosen so that the
C
For example:
Choose 22µF. If the output filter capacitor has signifi-
cant ESR, a zero occurs at:
If Z
with ceramic output capacitors. If Z
it should be cancelled with a pole set by capacitor C
connected from COMPSD to GND:
If C
Small, High-Efficiency, Five-Channel
Digital Still Camera Power Supply
20
IN
OUT
OUT
/(πL) = 214kHz, so choose f
ESR
P
______________________________________________________________________________________
)/4.7µH, or (3.35 - 1.5)/4.7µH = 394mA/µs.
is calculated to be < 10pF, it can be omitted.
OUT
R
R
C
LOAD
> f
C
C
= 1.5V:
C
= (1.25/1.5)(4.3/0.6) x (135µS/(6.28 x 40kHz)
= 3.2nF
= (1.25 x 0.6 x 0.25)/6.75µA = 27.8kΩ
C
= (V
OUT
, it can be ignored, as is typically the case
pole cancels the R
Z
FB
R
= 27kΩ x 3.3nF/4.3 = 20.7µF
ESR
C
C
/V
I
C
IND(PK)
= 0.6 I
OUT
P
OUT
= 1/(2π C
= C
R
C
)(R
LOAD
OUT
IND(PK)
that allows the required load
= 1.25 I
LOAD
C
C
R
OSC
= R
OUT
= 40kHz and calculate C
such that transient droop
ESR
/R
C
/6.75µA
OUT
C
= 440kHz, P
CS
C
/R
R
C
C
ESR
ESR
C
)(gm/2π f
C
C
zero:
to provide tran-
)
is less than f
IDEAL
IN
C
SLOPE
)
= 3.35V
is used,
IN
C
C
=
P
-
:
,
All MAX1565 auxiliary controllers drive external logic-
level N-channel MOSFETs. Significant MOSFET selec-
tion parameters are:
1) On-resistance (R
2) Maximum drain-to-source voltage (VDS(MAX))
3) Total gate charge (Q
4) Reverse transfer capacitance (CRSS)
DL_ swings between OUTSU and GND. Use a MOSFET
with on-resistance specified at or below the main output
voltage. The gate charge, Q
associated with charging the gate and helps to predict
MOSFET transition time between on and off states.
MOSFET power dissipation is a combination of
on-resistance and transition losses. The on-resistance
loss is:
where D is the duty cycle, I
current, and R
transition loss is approximately:
where V
inductor current, f
t
mately Q
I
power dissipation in the MOSFET is:
For most auxiliary applications, a Schottky diode rectifies
the output voltage. The Schottky diode’s low forward volt-
age 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.
The auxiliary controllers employ voltage-mode control
to regulate their output voltage. Optimum compensa-
tion somewhat depends on whether the design uses
continuous or discontinuous inductor current.
T
G
Auxiliary Controller Component Selection
is the transition time. The transition time is approxi-
is the gate drive current (typically 0.5A). The total
OUT
G
/I
G
P
P
, where Q
is the output voltage, I
MOSFET
TRANS
P
DS(ON)
RDSON
OSC
DS(ON)
= (V
= P
is the switching frequency, and
is MOSFET on-resistance. The
G
G
= D I
)
OUT
RDSON
is the total gate charge, and
)
G
L 2
L
, includes all capacitance
Auxiliary Compensation
I
L
is the average inductor
R
f
OSC
+ P
DS(ON)
External MOSFET
TRANS
t
L
T
)/3
is the average
Diode