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LTC1700

1700fa

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

■

High Efficiency: Up to 95%

■

No Current Sense Resistor Required

■

Constant Frequency 530kHz Operation Allows

Small Size, Surface Mount Inductors

■

OPTI-LOOP

Compensation Minimizes C

OUT

■

Selectable Burst Mode

Operation

■

Minimum Start-Up Voltage as Low as 0.9V

■

Synchronizable Between 400kHz and 750kHz

■

Micropower Shutdown: 10µA

■

Current Mode Operation for Excellent Line and Load

Transient Response

■

Soft-Start Reduces Supply Current Transients

■

1.5% Output Voltage Accuracy

■

Uses Low Value, Small Size, Surface Mount Inductors

■

Available in 10-Lead MSOP Package

The LTC

1700 is a current mode synchronous step-up

DC/DC controller that drives external N-channel and

P-channel power MOSFETs using a constant frequency

PWM architecture. Current limiting is provided by sensing

the voltage drop across the main MOSFET, eliminating the

need of a sense resistor. This No R

SENSE

technique helps

the LTC1700 maintain high efficiency at heavy loads while

Burst Mode operation ensures high efficiency at light loads,

thus providing high efficiencies over a wide range of load

currents.

The LTC1700 operates at a minimum input voltage as low as

0.9V. The device boasts a ±1.5% output voltage accuracy and

consumes only 200µA of quiescent current. In shutdown, it

only draws 10µA.

To prevent inductor current runaway, the duty cycle is limited

to 90%. Overvoltage protection is also provided which shuts

both the external MOSFETs off when tripped.

High constant operating frequency of 530kHz allows the use

of small inductors and output capacitors. The LTC1700 can

also be synchronized between 400kHz to 750kHz. Burst

Mode operation is inhibited when the device is externally

clocked or when the SYNC/MODE pin is pulled low to reduce

noise and RF interference.

■

Cellular Telephones

■

Wireless Modems

■

RF Communications

■

2.5V to 3.3V, 2.5V to 5V Converters

■

Battery-Powered Equipment

■

Telecom/Network Systems

No R

SENSE

Synchronous

Step-Up DC/DC Controller

Figure 1. High Efficiency Step-Up Converter

Efficiency, Power Loss vs Load Current

RUN/SS

SGND

SYNC/MODE

PGND

OUT

LTC1700

1.8µH

220pF

10µF

330µF

68µF

6.3V

0.1µF

22µF

2200Ω

316k

100k

3.3V to 4.2V

1700 • F01a

220pF

C1: CERAMIC TAIYO YUDEN LMK432BJ226MM

C2: AVX TAJB686K006R

L1: TOKO 919AS-IR8N (D104C TYPE)

C6: CERAMIC TAIYO YUDEN JMK316BJ106ML

C7: SANYO POSCAP 6TPB330M

M1: SILICONIX Si9804

M2: SILICONIX Si9803

LOAD CURRENT

EFFICIENCY (%)

100

0.001 0.1 1 2

1700 F01b

0.01

4.2V EFFICIENCY

3.3V EFFICIENCY

4.2V POWER

3.3V POWER

1.0

0.1

0.01

0.001

POWER LOSS (W)

, LTC and LT are registered trademarks of Linear Technology Corporation.

Burst Mode, OPTI-LOOP are registered trademarks of Linear Technology Corporation.

No R

SENSE

is a trademark of Linear Technology Corporation.

Summary of content (16 pages)

PAGE 1
LTC1700 No RSENSE Synchronous Step-Up DC/DC Controller U FEATURES DESCRIPTIO ■ The LTC®1700 is a current mode synchronous step-up DC/DC controller that drives external N-channel and P-channel power MOSFETs using a constant frequency PWM architecture. Current limiting is provided by sensing the voltage drop across the main MOSFET, eliminating the need of a sense resistor.
PAGE 2
LTC1700 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) Output Supply Voltage (VOUT) .....................– 0.3V to 6V RUN/SS, VFB Voltages ..............................– 0.3V to 2.4V SYNC/MODE, ITH Voltages ...........................– 0.3V to 6V SWITCH Voltage (SW) ..............................– 0.3V to 6.5V TG, BG Peak Output Current (<10µs) ......................... 1A Operating Temperature Range (Note 2) ...–40°C to 85°C Junction Temperature (Note 3) ...................
PAGE 3
LTC1700 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VOUT = 3V unless otherwise specified.
PAGE 4
LTC1700 U W TYPICAL PERFORMANCE CHARACTERISTICS Normalized Oscillator Frequency vs Temperature Minimum Operating Voltage vs Temperature 1.08 1.4 5 1.06 1.2 4 3 2 1 START-UP VOLTAGE (V) 6 NORMALIZED FREQUENCY RUN/SS CURRENT (µA) RUN/SS Current vs Temperature 1.04 1.02 1.00 0.98 5 25 45 65 85 105 125 TEMPERATURE (°C) 0.94 – 55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 1.0 Maximum Current Sense Voltage vs Temperature 0.8 2.22 0.6 2.
PAGE 5
LTC1700 U W TYPICAL PERFORMANCE CHARACTERISTICS Load Step Transient Response Burst Mode Operation Inhibited Load Step Transient Response Burst Mode Operation Enabled Load Step Transient Response Burst Mode Operation Inhibited A/C COUPLED OUTPUT VOLTAGE (0.2V/DIV) A/C COUPLED OUTPUT VOLTAGE (0.2V/DIV) A/C COUPLED OUTPUT VOLTAGE (0.2V/DIV) INDUCTOR CURRENT (2A/DIV) INDUCTOR CURRENT (2A/DIV) INDUCTOR CURRENT (2A/DIV) 1700 G13 VIN = 4.2V VOUT = 5V 2A/DIV VSYNC = VIN LOAD STEP = 100mA TO 1.
PAGE 6
LTC1700 W FUNCTIONAL DIAGRA U U VCC Y = “0” ONLY WHEN X IS A CONSTANT “1”, OTHERWISE Y = “1” Y BURST INHIBIT X SYNC/ MODE SLOPE COMP 5 MAIN OSC ITH 0.36V 90% DUTY CYCLE LIMIT 2 – VFB 3 + – EA 1.205V + – Ω gm = 0.9m SLEEP VOUT BURST 0.12V + 6 TG VOUT VOUT 7 1.205V REFERENCE VREF + 58mV RUN/ SOFT-START – OV VFB 3.8µA RUN/SS 4 + SWITCHING LOGIC AND BLANKING CIRCUIT VOUT 8 BG SGND 1 – + SC 9 PGND VOUT = “1” WHEN VOUT < 2.
PAGE 7
LTC1700 U OPERATIO output feedback voltage VFB. When the load current increases, it causes a slight decreases in VFB relative to the 1.205V reference, which in turn causes the ITH voltage to increase until the average inductor current matches the new load current. The internal oscillator can be synchronized to an external clock applied to the SYNC/MODE pin and can lock to a frequency between 400kHz to 750kHz. When not synchronized, the oscillator runs at 530kHz.
PAGE 8
LTC1700 U W U U APPLICATIONS INFORMATION The LTC1700 requires two external power MOSFETs, one for the main switch (N-channel) and one for the synchronous rectifier (P-channel). Since the voltage operating range of the LTC1700 is limited to less than 6V, the breakdown voltage of the MOSFETs is not a concern.
PAGE 9
LTC1700 U U W U APPLICATIONS INFORMATION load current. When the LTC1700 is operating in continuous mode, the duty cycles for the MOSFETs are: Main MOSFET Duty Cycle = 1 – VIN/VOUT then there is a net positive amount of energy stored in the output capacitor for every cycle. The output voltage then rises and once it exceeds 2.3V, the LTC1700 will successfully exit out of its start-up mode.
PAGE 10
LTC1700 U W U U APPLICATIONS INFORMATION To eliminate this subharmonic oscillation, a compensating ramp is added internally to the LTC1700 on the inductor current waveform when the duty cycle exceeds 5%. This scheme, known as slope compensation, makes the loop perceive that there is more inductor current than it actually has. As a result, the maximum current capability of the regulator is reduced. This reduction is proportional to the duty cycle and is shown in Figure 5.
PAGE 11
LTC1700 U W U U APPLICATIONS INFORMATION preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard”, which means that inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and output voltage ripple. Do not allow the core to saturate! Molypermalloy (from Magnetics, Inc.
PAGE 12
LTC1700 U W U U APPLICATIONS INFORMATION The efficiency of a switching regulator is equal to the output power divided by the input power (× 100%). Percent efficiency can be expressed as: % Efficiency = 100%–(L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement.
PAGE 13
LTC1700 U W U U APPLICATIONS INFORMATION where: VD = Voltage drop of P-channel parasitic diode IOUT = Initial load current during start-up COUT = Output capacitance RDS(ON) = (13.2)(0.9) = 11.9mΩ Hence you would select the start-up capacitor, CSS, to ensure tDELAY > tPOWERUP. Remember that the above equation is only valid for VIN < 2.3V. If VIN is greater than 2.3V, then tPOWERUP = 0ns. Design Example Assume the LTC1700 is used to convert a 3.3V input to 5V output.
PAGE 14
LTC1700 U U W U APPLICATIONS INFORMATION L1 3.2µH 1 3.9nF 5k 2 270pF SGND ITH 8 BG + C2 68µF 6.3V M2 Si9803 10 SW C1 22µF C3 22µF M1 IR7811W VOUT 5V/3A C4 470µF 6.3V ×2 + LTC1700 470pF 4 100k 3 5 316k RUN/SS 9 PGND VFB VIN 3.3V 6 TG 7 SYNC/MODE VOUT C1, C3: TAIYO YUDEN CERAMIC JMK316BJ106ML L1: SUMIDA CEP1233R2 C2: AVX TAJB68K006R M1: INTERNATIONAL RECTIFIER IR7811W C4: SANYO POSCAP 6TPB470M M2: SILICONIZ Si9803 1700 • F07 Figure 7.
PAGE 15
LTC1700 U TYPICAL APPLICATIO LTC1700 3.3V/1A Regulator with External Frequency Synchronization L1 1.5µH 1 100pF SGND 10 2 470pF 4 30k 3 5 53.6k ITH C4 22µF BG RUN/SS PGND VFB TG SYNC/MODE VOUT 8 + C1 68µF 6.3V + C3 220µF 6.3V M1 LTC1700 180pF 2.2k SW C2 10µF VIN 2V TO 2.4V 3.
PAGE 16
LTC1700 U TYPICAL APPLICATIO LTC1700 2.5V VIN 3.3V/1.8A Output Regulator L1 2.2µH 1 300pF SGND 2 470pF 4 30k 3 5 53.6k ITH C3 22µF RUN/SS BG PGND VFB TG SYNC/MODE VOUT C1: TAIYO YUDEN CERAMIC JMK316BJ106ML C2: AVX TAJB68K006R C3: TAIYO YUDEN CERAMIC JMK325BJ226M C4: KEMET T520D227M006AS 8 + M2 10 LTC1700 470pF 33k SW C1 10µF + C2 68µF 6.3V VIN 2.5V VOUT 3.3V/1.8A C4 220µF 6.