LTC3785-1 Linear Technology, LTC3785-1 Datasheet
LTC3785-1
Related parts for LTC3785-1
LTC3785-1 Summary of contents
Page 1
... Above, The LTC IN controller that drives all N-channel power MOSFETs from input voltages above, below and equal to the output volt- age. With an input range of 2.7V to 10V, the LTC3785-1 is well suited for a wide variety of single or dual cell Li-Ion or multi-cell alkaline/NiMH applications. TM SENSE The operating frequency can be programmed from 100kHz to 1MHz ...
Page 2
... LTC3785-1 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage (V ) ......................... –0.3V to 11V .............................................. –0.3V to 11V SVOUT SVIN SW1, SW2 Voltage: SSW1 SSW2 DC ............................................................. –1V to 11V Pulsed, <1μs ............................................. –2V to 12V RUN/SS, MODE, CCM DRV CC PGOOD Voltage ............................................ –0. TG1, V Voltages ................................... –0.3V to 16V BST1 With Respect to SW1 ............................... – ...
Page 3
... Note 3: Specifi cation is guaranteed by design and not 100% tested in production. Note 4: Current measurements are performed when the outputs are not switching. Note 5: The IC is tested in a feedback loop to make the measurement. LTC3785 3.6V 49.9k 59k. ...
Page 4
... LTC3785-1 TYPICAL PERFORMANCE CHARACTERISTICS Li-Ion to 3.3V Effi ciency vs Load Current 100 Burst Mode 90 OPERATION 80 70 FIXED 60 FREQUENCY MOSFET Si7940 L = 4.7μH WURTH WE- 500kHz OSC 0 0.0001 0.001 0.01 0.1 LOAD CURRENT (A) Burst Mode Ripple V OUT 50mV/DIV AC COUPLED INDUCTOR CURRENT 1A/DIV V = 3.3V 5μs/DIV ...
Page 5
... When driven high, the reverse current limit is set to the similar value of the forward current limit set by the I RT (Pin 7): Oscillator Programming Pin. A resistor from this pin to GND sets the free-running frequency of the IC. ≅ 2.5e10/RT. f OSC LTC3785-1 OV and UV Thresholds vs Temperature 8 OV THRESHOLD 6 4 ...
Page 6
... LTC3785-1 PIN FUNCTIONS MODE (Pin 8): Burst Mode Control Pin. • MODE = High: Enable Burst Mode Operation. In Burst Mode operation the operation is variable frequency, which provides a signifi cant effi ciency improvement at light loads. The Burst Mode operation will continue until the pin is driven low. ...
Page 7
... DISABLE + REVERSE LIMIT – REVERSE CURRENT LIMIT (ZERO LIMIT FOR BURST) DDRV BURST BURST LOGIC CDRV COMP COMP UVLO PGOOD GND/PGND 9 25 LTC3785-1 100% DUTY CHARGE PUMP SVIN 22 TG1 BST1 21 X10 – SW1 19 I SSW1 SAMPLED ...
Page 8
... POWER SWITCH CONTROL Figure 1 shows a simplifi ed diagram of how the four power switches are connected to the inductor, V Figure 2 shows the regions of operation for the LTC3785 function of duty cycle D. The power switches are properly controlled so that the transfer between modes is continuous. ...
Page 9
... V REGULATOR CC An internal P-channel low dropout regulator produces 4.35V at the ap- the drivers and internal circuitry of the LTC3785-1. The C V pin regulator can supply a peak current of 100mA and CC must be bypassed to ground with a minimum of 4.7μF placed directly adjacent to the V bypassing is necessary to supply the high transient cur- rent required by the MOSFET gate drivers and to prevent interaction between channels ...
Page 10
... FORWARD CURRENT LIMIT The LTC3785-1 is designed to sense the input current by sampling the voltage across MOSFET A during the on time of the switch (TG1 = High). The sense pins are I A current sense resistor can be used if increased accuracy is required ...
Page 11
... REVERSE CURRENT LIMIT comparator is tripped and The LTC3785-1 can be programmed to provide full class D operation or allowed to source and sink current equal to the current limit set value. This is achieved by asserting a high level on the CCM pin. To minimize the reverse output current, the CCM pin should be driven low or strapped to ground. During this mode only, – ...
Page 12
... LTC3785-1 APPLICATIONS INFORMATION INDUCTOR SELECTION The high frequency operation of the LTC3785-1 allows the use of small surface mount inductors. The inductor current ripple is typically set 20% to 40% of the maximum inductor current. For a given ripple the inductance terms are given as follows • ...
Page 13
... I . The drive voltage is set by the 4.35V V DS(MAX) Consequently, logic-level threshold MOSFETs must be used in LTC3785-1 applications. If the input voltage is expected to drop below 5V, then sub-logic threshold MOSFETs should be considered. In order to select the power MOSFETs, the power dissipated by the device must be known. For switch A, the maximum power dissipation happens in boost mode, when it remains on all the time ...
Page 14
... It is often useful to analyze individual losses to determine what is limiting the effi ciency and which change would produce the most improvement. Although all dissipative 37851 F05 elements in circuits produce losses, four main sources account for most of the losses in LTC3785-1 application circuits π 2 • • • ...
Page 15
... MOSFET in a surface mount package with 25mΩ at 2.5V and a total gate charge of 12nC. Checking the power dissipation of each switch will ensure reliable operation since the thermal resistance of the package is 60°C/W. = 2.7V to 10V (3. 3.3V (5%), OUT LTC3785 • – . ...
Page 16
... Place C area. Place C compact area. • Use immediate vias to connect the components (includ- ing the LTC3785-1’s GND/PGND pin) to the ground plane. Use several large vias for each power component. • Use planes for V fi ltering and to keep power losses low. ...
Page 17
... TG1 and • Connect FB and V BST1 BST2 terminals of C decoupling capacitor is used, it should be as close as IN possible to the LTC3785-1 GND pin. • Route I SVIN trace spacing. Ensure accurate current sensing with Kel- vin connections across MOSFET A or sense resistor. • Route I SVOUT PC trace spacing ...
Page 18
... V IN 2.7V TO 10V C VCC 4.7μ RUN/ SVIN V TG1 MA SENSE CMDSH BST1 A 0.22μF SW1 I SSW1 V DRV FB BG1 MB LTC3785 SVOUT TG2 MD PGOOD CMDSH-3 MODE V BST2 C B 0.22μF SW2 I I LSET SSW2 CCM BG2 MC GND 9V REGULATED WALL ADAPTER + Li-Ion 2. 1/2 Si7940DY 22μ ...
Page 19
... UF Package 24-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1697) 0.70 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC 0.75 0.05 4.00 0.10 (4 SIDES) 2.45 0.10 (4-SIDES) 0.200 REF 0.00 – 0.05 LTC3785-1 BOTTOM VIEW—EXPOSED PAD PIN 1 NOTCH R = 0.20 TYP 0.115 0.35 45° CHAMFER TYP 23 24 0.40 0. (UF24) QFN 0105 0.25 0.05 0.50 BSC ...
Page 20
... V IN 2.7V TO 10V C VCC 4.7μ RUN/ SVIN V TG1 MA SENSE CMDSH BST1 A 0.22μF SW1 I SSW1 V DRV FB BG1 MB LTC3785 SVOUT TG2 MD PGOOD CMDSH-3 MODE V BST2 C B 0.22μF SW2 I I LSET SSW2 CCM BG2 MC GND COMMENTS V : 2.4V to 5.5V Package V : 2.7V to 5.5V Amplifi ...