LTC1538-AUX_15 Datasheet, PDF (16/32 Pages) Linear Technology – Dual High Efficiency, Low Noise, Synchronous Step-Down Switching Regulators

LTC1538-AUX/LTC1539
APPLICATIONS INFORMATION
LTC1538-AUX
LTC1539*
+
V_IN
TGL1
EXTV_CC
TGS1*
R6
SFB1
SW1
R5
BG1
SGND PGND
V_IN
C_IN
N-CH _•
N-CH
N-CH
1N4148 _VSEC
L1
+
1:1
R_SENSE
•
+
1µF
V_OUT
C_OUT
OPTIONAL EXTV_CC
CONNECTION
5V ≤ V_SEC ≤ 9V
1538 F04a
*TGS1 ONLY AVAILABLE ON THE LTC1539
Figure 4a. Secondary Output Loop and EXTV_CC Connection
LTC1538-AUX
LTC1539*
+
V_IN
TGL1
TGS1*
EXTV_CC
SW1
BG1
PGND
V_IN BAT85
C_IN
N-CH
N-CH
L1
N-CH
*TGS1 ONLY AVAILABLE ON THE LTC1539
+
0.22µF
1µF
BAT85
VN2222LL
R_SENSE
+
BAT85
V_OUT
C_OUT
1538 F04b
Figure 4b. Capacitive Charge Pump for EXTV_CC
greater than 4.8V. This can be done with either the
inductive boost winding as shown in Figure 4a or the
capacitive charge pump shown in Figure 4b. The charge
pump has the advantage of simple magnetics.
4. EXTV_CC connected to an external supply. If an external
supply is available in the 5V to 10V range (EXTV_CC ≤ V_IN)
it may be used to power EXTV_CC providing it is compat-
ible with the MOSFET gate drive requirements. When
driving standard threshold MOSFETs, the external sup-
ply must be always present during operation to prevent
MOSFET failure due to insufficient gate drive. Note: care
must be taken when using the connections in items 3 or
4. These connections will effect the INTV_CC voltage
when either or both controllers are on.
Topside MOSFET Driver Supply (C_B,D_B)
External bootstrap capacitors C_B connected to the BOOST
1 and BOOST 2 pins supply the gate drive voltages for the
topside MOSFETs. Capacitor C_B in the Functional Diagram
is charged through diode D_B from INTV_CC when the
SW1(SW2) pin is low. When one of the topside MOSFETs
is to be turned on, the driver places the C_B voltage across
the gate source of the desired MOSFET. This enhances the
MOSFET and turns on the topside switch. The switch node
voltage SW1(SW2) rises to V_IN and the BOOST 1(BOOST
2) pin follows. With the topside MOSFET on, the boost
voltage is above the input supply: V_BOOST = V_IN + V_INTVCC.
The value of the boost capacitor C_B needs to be 100 times
that of the total input capacitance of the topside MOSFET(s).
The reverse breakdown on D_B must be greater than
V_IN(MAX).
Output Voltage Programming
The LTC1538-AUX/LTC1539 have pin selectable output
voltage programming. The output voltage is selected by
the V_PROG1 (V_PROG2) pin as follows:
V_PROG1,2 = 0V
V_PROG1,2 = INTV_CC
V_PROG2 = Open (DC)
V_OUT1,2 = 3.3V
V_OUT1,2 = 5V
V_OUT2 = Adjustable
The top of an internal resistive divider is connected to
SENSE^– 1 pin in Controller 1. For fixed output voltage
applications the SENSE^– 1 pin is connected to the output
voltage as shown in Figure 5a. When using an external
resistive divider for Controller 2, the V_PROG2 pin is left open
V_PROG1
SENSE ^– 1
LTC1538-AUX
LTC1539
SGND
GND: V_OUT = 3.3V
INTV_CC: V_OUT = 5V
+
V_OUT
C_OUT
1538 F05a
Figure 5a. LTC1538-AUX/LTC1539 Fixed Output Applications
1.19V ≤ V_OUT ≤ 9V
V_PROG2*
V_OSENSE2
LTC1538-AUX
LTC1539
SGND
*LTC1539 ONLY
OPEN (DC)
R2
100pF
R1
1538 F05b
( ) ^VOUT ^{= 1.19V}
1+
R2
R1
Figure 5b. LTC1538-AUX/LTC1539 Adjustable Applications
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