LTC3440 — 600mA Synchronous Buck-Boost DC-DC Converter

Overview

The LTC3440 (commonly packaged in 8-pin MSOP LTC3440EMS8) is a high-efficiency $600\text{ mA}$ synchronous buck-boost DC-DC converter manufactured by Analog Devices (originally Linear Technology). Operating over an input voltage range of $2.5\text{ V}$ to $5.5\text{ V}$ , it seamlessly maintains a fixed output voltage (such as $3.3\text{ V}$ ) regardless of whether the input voltage is higher, lower, or equal to the output voltage.

Integrating four internal low- $R_{DS(ON)}$ N-channel and P-channel MOSFET switches, the LTC3440 eliminates external Schottky diodes and achieves conversion efficiency up to $96\%$ . Its programmable switching frequency ( $300\text{ kHz}$ to $2.0\text{ MHz}$ ) makes it ideal for single-cell Li-Ion or multi-cell alkaline battery powered electronics.

Quick reference


Regulator Type	Synchronous Buck-Boost DC-DC Converter
Package	MSOP-8 / 10-Lead DFN ( $3\times3\text{ mm}$ )
Input Voltage Range ( $V_{IN}$ )	$2.5\text{ V}$ to $5.5\text{ V}$ DC
Output Voltage Range ( $V_{OUT}$ )	$2.5\text{ V}$ to $5.5\text{ V}$ DC (Adjustable)
Continuous Output Current ( $I_{OUT}$ )	Up to $600\text{ mA}$ ( $V_{IN} \ge 2.7\text{V}$ )
Switching Frequency	Programmable $300\text{ kHz} \dots 2.0\text{ MHz}$
Conversion Efficiency	Up to $96\%$
Operating Modes	Burst Mode® operation ( $25\ \mu\text{A}$ $I_Q$ ), Fixed Frequency PWM

Pinout (MSOP-8 Package)

text

        ┌─────────────┐
   SW1 ─│ 1         8 │─ Rt/SYNC
   VIN ─│ 2         7 │─ SHDN/MODE
   GND ─│ 3         6 │─ FB
   SW2 ─│ 4         5 │─ VOUT
        └─────────────┘

Pin	Name	Description
1	`SW1`	Switch node 1 connecting internal switches A & B to inductor terminal 1
2	`VIN`	Power supply input voltage (+2.5V to +5.5V DC)
3	`GND`	Ground reference (0 V)
4	`SW2`	Switch node 2 connecting internal switches C & D to inductor terminal 2
5	`VOUT`	Regulated DC output voltage pin
6	`FB`	Voltage feedback sense input ( $1.22\text{V}$ reference)
7	`SHDN`/`MODE`	Shutdown & mode selection pin (Low = Shutdown, High = PWM, Mid-voltage = Burst Mode)
8	`Rt` / `SYNC`	Frequency setting resistor pin (resistor to GND sets $f_{SW}$ or external clock sync)

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Input Supply Voltage	$V_{IN}$	2.5	3.7	5.5	V	Single Li-Ion / battery input
Feedback Reference Voltage	$V_{FB}$	1.196	1.220	1.244	V	$T_A = 25^\circ\text{C}$
Output Current Capability	$I_{OUT}$	600	—	—	mA	$V_{IN} \ge 2.7\text{V}, V_{OUT} = 3.3\text{V}$
Switch $R_{DS(ON)}$ (P-Channel)	$R_{DS(ON)P}$	—	0.19	—	$\Omega$	$V_{IN} = 3.3\text{V}$
Switch $R_{DS(ON)}$ (N-Channel)	$R_{DS(ON)N}$	—	0.22	—	$\Omega$	$V_{IN} = 3.3\text{V}$
Burst Mode Quiescent Current	$I_Q$	—	25	50	$\mu\text{A}$	Burst Mode enabled ( $V_{IN} = 3.3\text{V}$ )
Shutdown Current	$I_{SD}$	—	0.1	1.0	$\mu\text{A}$	$V_{SHDN} = 0\text{V}$

Typical Application Circuit (3.3V Output from Single Li-Ion Cell)

text

       +V_IN (2.5V - 5.5V Single Li-Ion Battery)
          │
       [Pin 2: VIN]
        LTC3440 ──── Pin 1: SW1 ─── [ 4.7µH Inductor ] ─── Pin 4: SW2 ──── LTC3440 ──── [Pin 5: VOUT] ─── +3.3V Output
          │                                                                                 │
       [Pin 3: GND]                                                                      [ R1 ]
          │                                                                                 │
         GND ───────────────────────────────────────────────────────────────────────────────┴──── [Pin 6: FB]
                                                                                            │
                                                                                          [ R2 ]
                                                                                            │
                                                                                           GND

$V_{OUT} = 1.22\text{V} \times \left( 1 + \frac{R_1}{R_2} \right)$

Common mistakes

Incorrect Inductor Placement across SW1 and SW2: A single inductor must be connected directly between Pin 1 (SW1) and Pin 4 (SW2). It is NOT connected to ground or output directly.
Noise on the feedback pin (FB): Place feedback resistors $R_1$ and $R_2$ as close to Pin 6 (FB) as possible, routing the trace away from noisy switch nodes SW1 and SW2.

Notes

Buck-Boost Seamless Operation: Unlike separate buck or boost switchers, LTC3440 automatically transitions between Buck, Buck-Boost, and Boost operating modes as a Li-Ion battery discharges from 4.2V down to 3.0V.

Interface	DC Input, DC Output, Dual Switch Nodes SW1 & SW2
Package	MSOP-8 / 10-Lead DFN