Datasheet

Output Capacitor Selection
-
D = ×
×
S IN out
C
S S out
V V I
V
V f C
_ ( ) _
D = ×
C ESR L peak C ESR
V I R
(10)
Operating Mode (MODE)
TPS61086
www.ti.com ................................................................................................................................................................................................ SLVSA05 – AUGUST 2009
For best output voltage filtering a low ESR output capacitor like ceramic capcaitor is recommended. Two to four
10 µ F ceramic output capacitors (or two 22 µ F) work for most of the applications. Higher capacitor values can be
used to improve the load transient response. Refer to Table 4 for the selection of the output capacitor.
Table 4. Rectifier Input and Output Capacitor Selection
CAPACITOR/SI VOLTAGE RATING SUPPLIER COMPONENT CODE
ZE
C
IN
22 µ F/1206 16 V Taiyo Yuden EMK316 BJ 226ML
IN bypass 1 µ F/0603 16 V Taiyo Yuden EMK107 BJ 105KA
C
OUT
10 µ F/1206 25 V Taiyo Yuden TMK316 BJ 106KL
To calculate the output voltage ripple, the following equation can be used:
with
Δ V
C
Output voltage ripple dependent on output capacitance,output current and switching frequency
V
S
Output voltage
V
IN
Minimum input voltage of boost converter
f
S
Converter switching frequency (typically 1.2 MHz)
I
out
Output capacitance
Δ V
C_ESR
Output voltage ripple due to output capacitors ESR (equivalent series resistance)
I
SWPEAK
Inductor peak switch current in the application
R
C_ESR
Output capacitors equivalent series resistance (ESR)
Δ V
C_ESR
can be neglected in many cases since ceramic capacitors provide very low ESR.
Power Save Mode
Connecting the MODE pin to GND (or any low logic level) enables the Power Save Mode operation. The
converter operates in quasi fixed frequency PWM (Pulse Width Modulation) mode at moderate to heavy load and
in the PFM (Pulse Frequency Modulation) mode during light loads, which maintains high efficiency over a wide
load current range.
In PFM mode the converter is skipping switch pulses. However, within a PFM pulse, the switching frequency is
still fixed to 1.2 MHz typically and the duty cycle determined by the input and output voltage. Therefore, the
inductor peak current will remain constant for a defined application. With an increasing output load current, the
PFM pulses become closer and closer (the PFM mode frequency gets higher) until no pulse is skipped anymore:
the device operates then in CCM (Continuous Conduction Mode) with normal PWM mode.
The PFM mode frequency (between each PFM pulse) depends on the load current, the external components like
the inductor or the output capacitor values as well as the output voltage. The device enters Power Save Mode as
the inductor peak current falls below a 0.6A typically and switches until V
S
is 1% higher than its nominal value.
The converter stops switching when V
S
= V
S
+ 0.5%. The output voltage will thenrefore oscillate between 0.5%
and 1% more than its nominal value which will provide excellent transient response to sudden load change, since
the output voltage drop will be reduced due to this slight positive offset (see Figure 9 ).
Forced PWM Mode
Pulling the MODE pin high forces the converter to operate in a continuous PWM mode evan at light load
currents. The advantage is that the converter operates with a quai constant frequency that allows simple filtering
of the swithcing frequency for noise-sensitive applications. In this mode and at light load, the efficiency is lower
compared to the Power Save Mode.
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