Rainbow Electronics MAX1627 - Manual (Page 8)

MAX1626/MAX1627
EXT Drive Voltage Range
EXT swings from V+ to GND and provides the gate
drive for an external P-channel power MOSFET. A high-
er supply voltage increases the gate drive to the
MOSFET and reduces on-resistance (R
DS(ON) ). See
External Switching Transistor section.
Quiescent Current
The device’s typical quiescent current is 70µA.
However, actual applications draw additional current to
supply MOSFET switching currents, OUT pin current, or
external feedback resistors (if used), and both the diode
and capacitor leakage currents. For example, in the cir-
cuit of Figure 1, with V+ at 7V and V
OUT at 5V, typical
no-load supply current for the entire circuit is 84µA.
When designing a circuit for high-temperature opera-
tion, select a Schottky diode with low reverse leakage.
Shutdown Mode
When SHDN is high, the device enters shutdown mode.
In this mode, the feedback and control circuit, reference,
and internal biasing circuitry are turned off. EXT goes
high, turning off the external MOSFET. The shutdown
supply current drops to less than 1µA. SHDN is a logic-
level input. Connect SHDN to GND for normal operation.
Reference
The 1.3V reference is suitable for driving external loads,
such as an analog-to-digital converter. It has a guaran-
teed 10mV maximum load regulation while sourcing load
currents up to 100µA. The reference is turned off during
shutdown. Bypass the reference with 0.1µF for normal
operation. Place the bypass capacitor within 0.2 inches
(5mm) of REF, with a direct trace to GND (Figure 7).
Soft-Start
Soft-start reduces stress and transient voltage slumps
on the power source. When the output voltage is near
ground, the minimum off-time is lengthened to limit peak
switching current. This compensates for reduced nega-
tive inductor current slope due to low output voltages.
________________Design Information
Setting the Output Voltage
The MAX1626’s output voltage can be selected to 3.3V
or 5V under logic control by using the
3/5 pin. The 3/5
pin requires less than 0.5V to ensure a 3.3V output, or
more than (V+ - 0.5)V to guarantee a 5V output. The
voltage sense pin (OUT) must be connected to the out-
put for the MAX1626.
The MAX1627’s output voltage is set using two resis-
tors, R2 and R3 (Figure 5), which form a voltage divider
between the output and GND. R2 is given by:
where V REF = 1.3V. Since the input bias current at FB
has a maximum value of 50nA, large values (10k
Ω
to
200k
Ω
) can be used for R3 with no significant accuracy
loss. For 1% error, the current through R2 should be at
R2 = R3 x
V
V
OUT
REF
−

 

1
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
8 _______________________________________________________________________________________
10 µ s/div
CIRCUIT OF FIGURE 1, V+ = 8V, V OUT = 5V, LOAD = 100mA
A: MOSFET DRAIN, 5V/div
B: OUT, 50mV/div, 5V DC OFFSET
C: INDUCTOR CURRENT, 1A/div
B
0A
C
A
Figure 3. Discontinuous-Conduction Mode, Light-Load-Current
Waveform
10 µ s/div
CIRCUIT OF FIGURE 1, V+ = 8V, V OUT = 5V, LOAD = 1.5A
A: MOSFET DRAIN, 5V/div
B: OUT, 50mV/div, 5V DC OFFSET
C: INDUCTOR CURRENT, 1A/div
B
0A
C
A
Figure 4. Continuous-Conduction Mode, Heavy-Load-Current
Waveform