MAX1640_09 Datasheet, PDF (9/12 Pages) Maxim Integrated Products – Adjustable-Output, Switch-Mode Current Sources with Synchronous Rectifier

Adjustable-Output, Switch-Mode
Current Source with Synchronous Rectifier
where V_TERM = 2V and V_OUT is the desired output
voltage.
Programming the Off-Time
When programming the off-time, consider such factors
as maximum inductor current ripple, maximum output
voltage, inductor value, and inductor current rating. The
output current ripple is less than the inductor current rip-
ple and depends heavily on the output capacitor’s size.
Perform the following steps to program the off-time:
1) Select the maximum output current ripple. I_R(A)
2) Select the maximum output voltage. V_OUT(MAX)(V)
3) Calculate the inductor value range as follows:
L_MIN = (V_OUTMAX x 1µs) / I_R
L_MAX = (V_OUTMAX x 10µs) / I_R
4) Select an inductor value in this range.
5) Calculate t_OFF as follows:
t_OFF = ^{L x IR}
V_OUTMAX
6) Program t_OFF by selecting R_TOFF from:
R_TOFF = (29.3 x 10⁹) x t_OFF
7) Calculate the switching frequency by:
fs = 1 / (t_ON + t_OFF)
where t_ON = (I_R x L) / (V_IN - V_OUT) and I_R = (V_OUT x
t_OFF) / L. L is the inductor value, V_IN is the input volt-
age, V_OUT is the output voltage, and I_R is the output
peak-to-peak current ripple.
Note that R_TOFF sets both the off-time and the pulse-
trickle charge period.
Reference
The on-chip reference is laser trimmed for a precise 2V
at REF. REF can source no more than 50µA. Bypass
REF with a 0.1µF capacitor to ground.
Constant-Current Loop: AC Loop
Compensation
The constant-current loop’s output is brought out at CC.
To reduce noise due to variations in switching currents,
bypass CC with a 1nF to 100nF capacitor to ground. A
large capacitor value maintains a constant average out-
put current but slows the loop response to changes in
switching current. A small capacitor value speeds up
the loop response to changes in switching current,
generating increased ripple at the output. Select C_CC
to optimize the ripple vs. loop response.
Synchronous Rectification
Synchronous rectification reduces conduction losses in
the rectifier by shunting the Schottky diode with a low-
resistance MOSFET switch. In turn, efficiency increases
by about 3% to 5% at heavy loads. To prevent cross-
conduction or “shoot-through,” the synchronous rectifier
turns on shortly after the P-channel power MOSFET
Table 2. Component Manufacturers
COMPONENT
Inductor
MOSFETs
Sense Resistor
Capacitors
Rectifier
MANUFACTURER
Sumida
CDRH125 series
Coilcraft
D03316P series
Coiltronics
UP2 series
International Rectifier IRF7309
Siliconix
S14539DY
Dale
WSL-2010 series
IRC
LR2010-01 series
AVX
TPS series
Sprague
595D series
Motorola
MBAR5340t3
IN5817-IN5822
Nihon
NSQ03A04
turns off. The synchronous rectifier remains off for 90%
of the off-time. In low-cost designs, the synchronous
rectifier FET may be replaced by a Schottky diode.
Component Selection
External Switching Transistors
The MAX1640/MAX1641 drive an enhancement-mode
P-channel MOSFET and a synchronous-rectifier N-
channel MOSFET (Table 2).
When selecting a P-channel FET, some important para-
meters to consider are on-resistance (r_DS(ON)), maxi-
mum drain-to-source voltage (V_DS max), maximum
gate-to-source voltage (V_GS max), and minimum
threshold voltage (V_TH min).
In high-current applications, MOSFET package power
dissipation often becomes a dominant design factor.
I2R power losses are the greatest heat contributor for
both high-side and low-side MOSFETs. Switching loss-
es affect the upper MOSFET only (P-channel), since the
Schottky rectifier or the N-FET body diode clamps the
switching node before the synchronous rectifier turns on.
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