Datasheet

Vout + 0.8 V Vin
D =
Vout + 0.8 V
-
2 2
min_on SW
out_skip
Vin T f
I =
2 (Vout + 0.8V Vin) L
´ ´
´ - ´
R1
Vout = 0.5 V +1
R2
Vout
R1 = R2 1
0.5 V
æ ö
´
ç ÷
è ø
æ ö
´ -
ç ÷
è ø
TPS61093
OUT
FB
R1
R2
Cff
Option
C4
VO
C2
OUT
FB
R1
R2
Cff
Option
VO
C2
TPS61093
(a)WithisolationFET (b)WithoutisolationFET
TPS61093
SLVS992A –SEPTEMBER 2009–REVISED NOVEMBER 2009 www.ti.com
APPLICATION INFORMATION
SWITCH DUTY CYCLE
The maximum switch duty cycle (D) of the TPS61093 is 90% (minimum). The duty cycle of a boost converter
under continuous conduction mode (CCM) is given by:
(1)
The duty cycle must be lower than the specification in the application; otherwise the output voltage cannot be
regulated.
The TPS61093 has a minimum ON pulse width once the PWM switch is turned on. As the output current drops,
the device enters discontinuous conduction mode (DCM). If the output current drops extremely low, causing the
ON time to be reduced to the minimum ON time, the TPS61093 enters pulse-skipping mode. In this mode, the
device keeps the power switch off for several switching cycles to keep the output voltage in regulation. See
Figure 11. The output current when the IC enters skipping mode is calculated with Equation 2.
(2)
Where
T
min_on
= Minimum ON pulse width specification (typically 65-ns);
L = Selected inductor value;
f
SW
= Converter switching frequency (typically 1.2-MHz)
OUTPUT PROGRAM
To program the output voltage, select the values of R1 and R2 (see Figure 13) according to Equation 3.
(3)
A recommended value for R2 is approximately 10-kΩ which sets the current in the resistor divider chain to
0.5V/10kΩ = 50-μA. The output voltage tolerance depends on the VFB accuracy and the resistor divider.
Figure 13. Resistor Divider to Program Output Voltage
10 Submit Documentation Feedback Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): TPS61093