Datasheet

1A, 76V, High-Efficiency MAXPower
Step-Down DC-DC Converter
Detailed Description
The MAX5035 step-down DC-DC converter operates
from a 7.5V to 76V input voltage range. A unique volt-
age-mode control scheme with voltage feed-forward
and an internal switching DMOS FET provides high effi-
ciency over a wide input voltage range. This pulse-
width modulated converter operates at a fixed 125kHz
switching frequency. The device also features automat-
ic pulse-skipping mode to provide low quiescent cur-
rent and high efficiency at light loads. Under no load,
the MAX5035 consumes only 270µA, and in shutdown
mode, consumes only 10µA. The MAX5035 also fea-
tures undervoltage lockout, hiccup mode output short-
circuit protection, and thermal shutdown.
Shutdown Mode
Drive ON/OFF to ground to shut down the MAX5035.
Shutdown forces the internal power MOSFET off, turns
off all internal circuitry, and reduces the V
IN
supply cur-
rent to 10µA (typ). The ON/OFF rising threshold is
1.69V (typ). Before any operation begins, the voltage at
ON/OFF must exceed 1.69V (typ). The ON/OFF input
has 100mV hysteresis.
Undervoltage Lockout (UVLO)
Use the ON/OFF function to program the UVLO thresh-
old at the input. Connect a resistive voltage-divider
from V
IN
to GND with the center node to ON/OFF as
shown in Figure 1. Calculate the threshold value by
using the following formula:
The minimum recommended V
UVLO(TH)
is 6.5V, 7.5V, and
13V for the output voltages of 3.3V, 5V, and 12V, respec-
tively. The recommended value for R2 is less than 1MΩ.
If the external UVLO threshold-setting divider is not
used, an internal undervoltage-lockout feature monitors
the supply voltage at V
IN
and allows operation to start
when V
IN
rises above 5.2V (typ). This feature can be
used only when V
IN
rise time is faster than 2ms. For
slower V
IN
rise time, use the resistive-divider at
ON/OFF.
Boost High-Side Gate Drive (BST)
Connect a flying bootstrap capacitor between LX and
BST to provide the gate-drive voltage to the high-side
N-channel DMOS switch. The capacitor is alternately
charged from the internally regulated output voltage VD
and placed across the high-side DMOS driver. Use a
0.1µF, 16V ceramic capacitor located as close to the
device as possible.
On startup, an internal low-side switch connects LX to
ground and charges the BST capacitor to VD. Once the
BST capacitor is charged, the internal low-side switch
is turned off and the BST capacitor voltage provides
the necessary enhancement voltage to turn on the
high-side switch.
Thermal-Overload Protection
The MAX5035 features integrated thermal overload pro-
tection. Thermal overload protection limits total power
dissipation in the device, and protects the device in the
event of a fault condition. When the die temperature
exceeds +160°C, an internal thermal sensor signals the
shutdown logic, turning off the internal power MOSFET
and allowing the IC to cool. The thermal sensor turns the
internal power MOSFET back on after the IC’s die tem-
perature cools down to +140°C, resulting in a pulsed
output under continuous thermal overload conditions.
Applications Information
Setting the Output Voltage
The MAX5035A/B/C have preset output voltages of 3.3V,
5.0V, and 12V, respectively. Connect FB to the preset
output voltage (see the
Typical Operating Circuit ).
The MAX5035D/E versions offer an adjustable output
voltage. Set the output voltage with a resistive voltage-
divider connected from the circuit’s output to ground
(Figure 1). Connect the center node of the divider to
FB. Choose R4 less than 15kΩ, then calculate R3 as
follows:
R
V
R
OUT
3
122
122
4=
−
×
(.)
.
V
R
R
V
UVLO TH()
.=+
⎛
⎝
⎜
⎞
⎠
⎟
×1
1
2
185
MAX5035D
GND
BST
LX
V
IN
SGND
FB
D1
50SQ100
VD
100µH
V
OUT
5V
V
IN
7.5V TO 76V
68µF
0.1µF
0.1µF
C
OUT
68µF
R1
R2
R3
41.2kΩ
R4
13.3kΩ
ON/OFF
Figure 1. Adjustable Output Voltage
MAX5035
10
Maxim Integrated