Rainbow Electronics MAX1813 - Manual (Page 15)

MAX1813
Dynamically-Adjustable, Synchronous Step-Down
Controller with Integrated Voltage Positioning
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(Figure 2). This architecture relies on the output ripple
voltage to provide the PWM ramp signal. Thus, the out-
put filter capacitor’s equivalent series resistance (ESR)
acts as a feedback resistor. The control algorithm is
simple: the high-side switch on-time is determined sole-
ly by a one-shot whose period is inversely proportional
to input voltage and directly proportional to output volt-
age (see On-Time One-Shot). Another one-shot sets a
minimum off-time (400ns typ). The on-time one-shot is
triggered if the error comparator is low, the low-side
switch current is below the current-limit threshold, and
the minimum off-time one-shot has timed out.
On-Time One-Shot (TON)
The heart of the PWM core is the one-shot that sets the
high-side switch on-time. This fast, low-jitter, adjustable
one-shot includes circuitry that varies the on-time in
response to the input and output voltages. The high-
side switch on-time is inversely proportional to V+ and
directly proportional to the output voltage as set by the
DAC code. This algorithm results in a nearly constant
switching frequency despite the lack of a fixed-frequen-
cy clock generator. The benefits of a constant switch-
ing frequency are twofold: first, the frequency can be
selected to avoid noise-sensitive regions such as the
455kHz IF band; second, the inductor ripple-current
operating point remains relatively constant, resulting in
COMPONENT
CIRCUIT 1
(FIGURE 1)
Output Voltage 0.6V to 1.75V
Input Voltage Range 7V to 24V
Maximum Load Current 22A
Inductor
0.68 µH
Sumida CDEP134H-0R6 or
Panasonic ETQP6F0R6BFA
Frequency 300kHz (TON = float)
High-Side MOSFET
International Rectifier
(2) IRF7811A
Low-Side MOSFET
Fairchild (3) FDS7764A or
International Rectifier
(3) IRF7822A
Input Capacitor
(6) 10 µF, 25V
Taiyo Yuden TMK432BJ106 or
TDK C4532X5R1E106M
Output Capacitor
(6) 220 µF
Panasonic
EEFUE0E221R
Current-Sense Resistor
1.5m Ω Dale WSL 2512, plus
0.5m Ω copper PC board trace
ILIM Level V CC (Default)
Voltage-Positioning Gain
Resistor
150k Ω
M A NUF ACT URER
PH ONE
[ C OUN TRY
CO DE]
WEBSITE
MOSFETs
Fairchild
Semiconductor [1] 888-
522-5372
www.fairchildsemi.com
International
Rectifier [1] 310-
322-3331
www.irf.com
Siliconix
[1] 203-
268-6261
www.vishay.com
Capacitors
Kemet
[1] 408-
986-0424
www.kemet.com
Panasonic
[1] 847-
468-5624
www.panasonic.com
Sanyo
[ 65] 281-
3226
( S i ng ap or e)
[ 1] 408-
749- 9714
www.secc.co.jp
Taiyo Yuden
[03] 3667-
3408
(Japan)
[1] 408-
573-4150
www.t-yuden.com
Inductors
Coilcraft
[1] 800-
322-2645
www.coilcraft.com
Coiltronics
[1] 561-
752-5000
www.coiltronics.com
Sumida
[1] 408-
982-9660
www.sumida.com
Table 1. Component Selection for
Standard Applications Table 2. Component Suppliers