FAN5336 Datasheet, PDF (7/9 Pages) Fairchild Semiconductor – 1.5MHz Boost Regulator with 33V Integrated FET Switch

Applications Information
Setting the Output Voltage
The internal feedback voltage reference (V_REF) is 1.23V
(typical). The output voltage is divided by a resistor
divider, R₁ and R₂ to the FB pin. The output voltage is
calculated by:
V_OUT
=
⎛
V_{R E F ⎜⎜} 1
⎝
+
R_{-----1-⎟⎞}
R_{2⎠⎟}
EQ 1
The maximum output current depends on the output volt-
age settings. Table 1 provides the recommended voltage
for several steady-state configurations:
Table 1: Recommended Voltages
V_OUT I_LOAD Max.
(V)
(mA)
R₁ (kΩ) for
R₂ = 10kΩ
9
50
63.2
V_IN Range
(V)
2.7 to 5.5
9
170
63.2
2.9 to 4.7
12
125
87.5
2.7 to 5.5
21
75
160
3.6 to 5.5
33
50
258
4.0 to 5.5
33
10
258
2.7 to 5.5
Inductor Selection
The inductor parameters directly related to device perfor-
mance are saturation current and DC resistance. The
FAN5336 operates with a typical inductor value of 6.8µH.
The lower the DC resistance, the higher the efficiency.
Balancing inductor size, cost, and overall efficiency
allows optimum choice.
The inductor saturation current should be rated around
1.5A, which is the threshold of the internal current limit
circuit. This limit is reached only during the start-up and
with heavy load conditions. When this occurs, the con-
verter can shift into discontinuous conduction mode due
to the automatic turn-off of the switching transistor,
resulting in higher ripple and reduced efficiency.
Table 2: Recommended Inductors
Inductor
Value Vendor
Part Number
Comment
6.8µH
TDK SLF7028T6R8M1R3
Lower
Profile
6.8µH Murata LQS66C6R8M04
Higher
Efficiency
Table 3: Recommended Capacitors
Cap. Value Vendor
Part Number
10µF
Murata GRM31CR70J106KA01B
4.7µF
Murata GRM32RR61E475KC31B
A feedforward capacitor (C_F) is required for stability. The
recommended value is around 180pF.
Diode Selection
The external diode used for rectification is usually a
Schottky diode. Its average forward current should
exceed the load current and its reverse voltage maxi-
mum ratings should exceed the voltage at the output of
the converter. A barrier Schottky diode, such as BAT54,
is preferred due to its low reverse current over the
temperature range.
Care should be taken to avoid any short circuit of V_OUT
to GND, even with the IC disabled, since the diode can
be instantly damaged by the excessive current.
Flash LED Driver
The FAN5336 may be used to drive a serial flash LED
with up to 100mA current at 21V for a maximum of
400ms on-time and 10% duty cycle.
Thermal Shutdown
When the die temperature exceeds 150°C, a reset
occurs and remains in effect until the die cools to 130°C,
when the circuit is allowed to restart.
PCB Layout Recommendations
The inherently high peak currents and switching fre-
quency of power supplies require careful PCB layout
design. For best results, use wide traces for high-current
paths and place the input capacitor, the inductor, and the
output capacitor as close as possible to the integrated
circuit terminals. The resistor divider that sets the output
voltage should be routed away from the inductor to avoid
RF coupling. A four-layer PCB, with at least one ground
plane connected to Pin 2 of the IC, is recommended. The
ground plane acts as an electromagnetic shield to
reduce EMI and parasitic coupling between components.
Capacitors Selection
For best performance, low-ESR input and output capaci-
tors are required. Ceramic capacitors of C_IN = 10µF and
C_OUT = 4.7µF, placed as close as possible to the IC pins,
are recommended for the lower input and output ripple.
The output capacitor voltage rating should be selected
according to the V_OUT setting.
Figure 11. Recommended PCB Layout
© 2004 Fairchild Semiconductor Corporation
FAN5336 Rev. 1.0.0
7
www.fairchildsemi.com