LT1054_15 Datasheet, PDF (10/33 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

LT1054
SLVS033G – FEBRUARY 1990 – REVISED JULY 2015
Feature Description (continued)
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R3
V_IN
1
FB/SD
8
V_CC
2.2 μF
+
2
CAP+
7
OSC
R4
C_IN
10-μF
+
Tantalum
LT1054
3
6
GND
V_REF
R1
4
CAP−
5
R2
V_OUT
Restart
Shutdown
For example: To get V_O = −5 V, referenced to the ground terminal of the LT1054
æ
ö
æ
ö
ç
R2 = R1ç
V_OUT
÷
ç
+ 1÷ = 20 kW _ç
-5 V
÷
+ 1_÷ = 102.6 kW †
çèç
V_REF
2
-
40
mV
÷ø÷
çèç
2.5 V
2
-
40
mV
÷ø÷
Where: R1 = 20 kΩ
V_REF = 2.5 V Nominal
V_OUT
† Choose the closest 1% value.
Figure 11. Basic Regulation Configuration
C1
C_OUT
100-μF
Tantalum
7.3.2 External Oscillator Synchronization
This pin can be used to raise or lower the oscillator frequency or to synchronize the device to an external clock.
Internally Pin 7 is connected to the oscillator timing capacitor (Ct ≈ 150pF) which is alternately charged and
discharged by current sources of ±7µA so that the duty cycle is ≈50%. The LT1054 oscillator is designed to run
in the frequency band where switching losses are minimized. However the frequency can be raised, lowered, or
synchronized to an external system clock if necessary.
1
FB/SD
8
C2
V_CC
V_IN
2
CAP+
7
OSC
LT1054
+
3
GND
6
V_REF
C1
4
CAP−
5
V_OUT
Figure 12. External-Clock System
The frequency can be lowered by adding an external capacitor (C1, Figure 12) from Pin 7 to ground. This will
increase the charge and discharge times which lowers the oscillator frequency. The frequency can be increased
by adding an external capacitor (C2, Figure 12, in the range of 5pF to 20pF) from Pin 2 to Pin 7. This capacitor
will couple charge into CT at the switch transitions, which will shorten the charge and discharge time, raising the
oscillator frequency. Synchronization can be accomplished by adding an external resistive pull-up from Pin 7 to
10
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