Datasheet
Iripple
2
Ipeak = Iload +
(Vin ± Vout) x Vout
fsw x L x Vin
Iripple =
R2 =
Vout
Vfb
©
§
¹
·
R1
-1
LM26001
SW
SW
BOOT
FB
COMP
VDD
GND
FPWM
FREQ
SS
SYNC
EN
PGOOD
VIN
2
3
13
9
144
10
6
11
5
8
7
15
16
VIN : 4V - 38V
VOUT : 3.3V
VIN
VBIAS
1 12
C1
3.3 PF
50V
C5
10 nF
R3
120k
1%
C8
4.7 nF
R5
15k
L
22 PH
3.5A
C4
0.1 PF
R2
33k
1%
R1
56k
1%
C6
100 PF
12 m:
C3
10 PF
R4
D1
3A
60V
VDD
17
EP
R6
10k
EN
SYNC
+
C2
47 PF
50V
PGOOD
+
C9
47 pF
200k
Ref # Manufacturer Part Number
C1
C2
C6
D1
L1
TDK
Panasonic
Panasonic
NIEC
TDK
3225JB1H335K
EEVFK1H470P
EEFVEOK101R
NSQ03A06
SLF12565T-
220M3R5
LM26001, LM26001Q
www.ti.com
SNVS430G –MAY 2006–REVISED MAY 2006
Figure 21. Example Circuit 1.5A Max, 305 kHz
SETTING OUTPUT VOLTAGE
The output voltage is set by the ratio of a voltage divider at the FB pin as shown in the typical application. The
resistor values can be determined by the following equation:
(9)
Where Vfb = 1.234V typically.
A maximum value of 150kΩ is recommended for the sum of R1 and R2.
As input voltage decreases towards the nominal output voltage, the LM26001 can skip up to seven off-pulses as
described in the LOW VIN OPERATION AND UVLO section. In low output voltage applications, if the on-time
reaches Ton
MIN
, the device will skip on-pulses to maintain regulation. There is no limit to the number of pulses
that are skipped. In this mode of operation, however, output ripple voltage may increase slightly.
INDUCTOR
The output inductor should be selected based on inductor ripple current. The amount of inductor ripple current
compared to load current, or ripple content, is defined as Iripple/Iload. Ripple content should be less than 40%.
Inductor ripple current, Iripple, can be calculated as shown below:
(10)
Larger ripple content increases losses in the inductor and reduces the effective current limit.
Larger inductance values result in lower output ripple voltage and higher efficiency, but a slightly degraded
transient response. Lower inductance values allow for smaller case size, but the increased ripple lowers the
effective current limit threshold.
Remember that inductor value also affects the sleep mode threshold as shown in Figure 17.
When choosing the inductor, the saturation current rating must be higher than the maximum peak inductor
current and the RMS current rating should be higher than the maximum load current. Peak inductor current,
Ipeak, is calculated as:
(11)
Copyright © 2006, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LM26001 LM26001Q