Rainbow Electronics MAX1708 User Manual
Page 10
MAX1708
High-Frequency, High-Power, Low-Noise,
Step-Up DC-DC Converter
10
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troller functional diagram. The MAX1708 enters syn-
chronized current-mode PWM when a clock signal
(350kHz < f
CLK
< 1MHz) is applied to CLK. For wire-
less or noise-sensitive applications, this ensures that
switching harmonics are predictable and kept outside
the IF frequency band(s). High-frequency operation
permits low-magnitude output ripple voltage and mini-
mum inductor and filter capacitor size. Switching loss-
es will increase at higher frequencies (see MAX1708 IC
Power Dissipation).
Setting the Output Voltage
The MAX1708 features Dual Mode™ operation. When
FB is connected to ground, the MAX1708 generates a
fixed output voltage of either +3.3V or +5V, depending
on the logic applied to the 3.3/5 input (Figure 1). The
output can be configured for other voltages, using two
external resistors as shown in Figure 4. To set the out-
put voltage externally, choose an R3 value that is large
enough to minimize load at the output but small enough
to minimize errors due to leakage and the time constant
to FB. A value of R4
≤
50k
Ω is required
where V
FB
= 1.24V.
Soft-Start/Current Limit Adjustment
(SS/LIM)
The soft-start pin allows the soft-start time to be adjust-
ed by connecting a capacitor from SS/LIM to GND.
Select capacitor C3 (see Figure 1):
t
SS
= 4ms + [110
✕
C3 (in µF)]
where t
SS
is the time (in milliseconds) it takes output to
reach its final value.
To improve efficiency or reduce inductor size at
reduced load currents, the current limit can be reduced
from its nominal value (see Electrical Characteristics).
A resistor (R1 in Figure 1) between SS/LIM and ground
reduces the current limit as follows:
where I
1
is the desired current limit in amperes and R1
≤ 312kΩ. I
LIM
= 5A, if R1 is omitted.
R
k
I
I
LIM
1
312
1
=
×
Ω
R
R
V
V
OUT
FB
3
4
1
=
−
ONA
ONB
MAX1708
0
0
On
0
1
Off
1
0
On
1
1
On
Figure 2. Simplified Functional Diagram
Table 2. Selecting Switching Frequency
Table 1. On/Off Logic Control
CLK
MODE
0
Not allowed
1
PWM
External clock
(350kHz
−1000kHz)
Synchronized PWM
N
PWM
CONTROLLER
STARTUP
OSCILLATOR
OSC
UNDERVOLTAGE LOCKOUT
FB
EN
D
EN
Q
600kHz
OSCILLATOR
EN
REFERENCE
ON
RDY
DUAL MODE
FB
LX
PGND
1.26V
2.15V
OUT
ONA
ONB
REF
CLK
FB
3.3/5
GND
IC POWER
MAX1708
OUT
Dual Mode is a trademark of Maxim Integrated Products.